DOC: remove obsolete section about header manipulation

Cyril Bont� reported that the doc contains two chapters number 6,
one of which is a leftover of the section about old header manipulation
directives that were removed by commit a6a56e6 ("MEDIUM: config: Remove
parsing of req* and rsp* directives"). This patch removes this.

doc/configuration.txt

@@ -12361,107 +12361,6 @@ timeout <event> <time>
      hold obsolete        30s
 
 
-6. HTTP header manipulation
----------------------------
-
-In HTTP mode, it is possible to rewrite, add or delete some of the request and
-response headers based on regular expressions. It is also possible to block a
-request or a response if a particular header matches a regular expression,
-which is enough to stop most elementary protocol attacks, and to protect
-against information leak from the internal network.
-
-If HAProxy encounters an "Informational Response" (status code 1xx), it is able
-to process all rsp* rules which can allow, deny, rewrite or delete a header,
-but it will refuse to add a header to any such messages as this is not
-HTTP-compliant. The reason for still processing headers in such responses is to
-stop and/or fix any possible information leak which may happen, for instance
-because another downstream equipment would unconditionally add a header, or if
-a server name appears there. When such messages are seen, normal processing
-still occurs on the next non-informational messages.
-
-This section covers common usage of the following keywords, described in detail
-in section 4.2 :
-
-  - reqadd     <string>
-  - reqallow   <search>
-  - reqiallow  <search>
-  - reqdel     <search>
-  - reqidel    <search>
-  - reqdeny    <search>
-  - reqideny   <search>
-  - reqpass    <search>
-  - reqipass   <search>
-  - reqrep     <search> <replace>
-  - reqirep    <search> <replace>
-  - reqtarpit  <search>
-  - reqitarpit <search>
-  - rspadd     <string>
-  - rspdel     <search>
-  - rspidel    <search>
-  - rspdeny    <search>
-  - rspideny   <search>
-  - rsprep     <search> <replace>
-  - rspirep    <search> <replace>
-
-With all these keywords, the same conventions are used. The <search> parameter
-is a POSIX extended regular expression (regex) which supports grouping through
-parenthesis (without the backslash). Spaces and other delimiters must be
-prefixed with a backslash ('\') to avoid confusion with a field delimiter.
-Other characters may be prefixed with a backslash to change their meaning :
-
-  \t   for a tab
-  \r   for a carriage return (CR)
-  \n   for a new line (LF)
-  \    to mark a space and differentiate it from a delimiter
-  \#   to mark a sharp and differentiate it from a comment
-  \\   to use a backslash in a regex
-  \\\\ to use a backslash in the text (*2 for regex, *2 for haproxy)
-  \xXX to write the ASCII hex code XX as in the C language
-
-The <replace> parameter contains the string to be used to replace the largest
-portion of text matching the regex. It can make use of the special characters
-above, and can reference a substring which is delimited by parenthesis in the
-regex, by writing a backslash ('\') immediately followed by one digit from 0 to
-9 indicating the group position (0 designating the entire line). This practice
-is very common to users of the "sed" program.
-
-The <string> parameter represents the string which will systematically be added
-after the last header line. It can also use special character sequences above.
-
-Notes related to these keywords :
----------------------------------
-  - these keywords are not always convenient to allow/deny based on header
-    contents. It is strongly recommended to use ACLs with the "block" keyword
-    instead, resulting in far more flexible and manageable rules.
-
-  - lines are always considered as a whole. It is not possible to reference
-    a header name only or a value only. This is important because of the way
-    headers are written (notably the number of spaces after the colon).
-
-  - the first line is always considered as a header, which makes it possible to
-    rewrite or filter HTTP requests URIs or response codes, but in turn makes
-    it harder to distinguish between headers and request line. The regex prefix
-    ^[^\ \t]*[\ \t] matches any HTTP method followed by a space, and the prefix
-    ^[^ \t:]*: matches any header name followed by a colon.
-
-  - for performances reasons, the number of characters added to a request or to
-    a response is limited at build time to values between 1 and 4 kB. This
-    should normally be far more than enough for most usages. If it is too short
-    on occasional usages, it is possible to gain some space by removing some
-    useless headers before adding new ones.
-
-  - keywords beginning with "reqi" and "rspi" are the same as their counterpart
-    without the 'i' letter except that they ignore case when matching patterns.
-
-  - when a request passes through a frontend then a backend, all req* rules
-    from the frontend will be evaluated, then all req* rules from the backend
-    will be evaluated. The reverse path is applied to responses.
-
-  - req* statements are applied after "block" statements, so that "block" is
-    always the first one, but before "use_backend" in order to permit rewriting
-    before switching.
-
-
 6. Cache
 ---------