Revert "MINOR: istbuf: add b_fromist() to make a buffer from an ist"

This reverts commit 9e46496d45. It was
wrong and is not reliable, depending on the compiler's version and
optimization, as the struct is assigned inside a statement, thus on
its own stack. It's not needed anymore now so let's remove this.

include/common/istbuf.h

@@ -33,9 +33,6 @@
 #include <common/ist.h>
 
 
-/* makes a buffer pointer from an IST */
-#define b_fromist(in) ({ const struct ist __i = (in); &(const struct buffer){ .area = __i.ptr, .head = 0, .data = __i.len, .size = __i.len}; })
-
 /* b_isteq() : returns > 0 if the first <n> characters of buffer <b> starting
  * at offset <o> relative to the buffer's head match <ist>. (empty strings do
  * match). It is designed to be used with reasonably small strings (it matches