CI: github-actions: remove obsolete options

2.5-dev1 removed http-use-htx but the h2spec config was not updated
accordingly, causing failures. In addition, let's also remove the
unneeded "nbthread 4" which is either too much or not enough (it's
automatic nowadays), and remove "option httplog" which causes a
warning since there's no defined log destination.

.github/h2spec.config

@@ -1,6 +1,5 @@
 global
     log stdout local0
-    nbthread 4
     tune.ssl.default-dh-param 2048
     tune.ssl.capture-cipherlist-size 1
 
@@ -15,8 +14,6 @@ defaults
     timeout http-request 10s
     timeout http-keep-alive 300s
 
-    option httplog
-    option http-use-htx
     option logasap
     option http-buffer-request