gskit: remove

We remove support for building curl with gskit.

 - This is a niche TLS library, only running on some IBM systems
 - no regular curl contributors use this backend
 - no CI builds use or verify this backend
 - gskit, or the curl adaption for it, lacks many modern TLS features
   making it an inferior solution
 - build breakages in this code take weeks or more to get detected
 - fixing gskit code is mostly done "flying blind"

This removal has been advertized in DEPRECATED in Jan 2, 2023 and it has
been mentioned on the curl-library mailing list.

It could be brought back, this is not a ban. Given proper effort and
will, gskit support is welcome back into the curl TLS backend family.

Closes #11460

lib/Makefile.inc

@@ -44,7 +44,6 @@ LIB_VAUTH_HFILES =      \
 
 LIB_VTLS_CFILES =           \
   vtls/bearssl.c            \
-  vtls/gskit.c              \
   vtls/gtls.c               \
   vtls/hostcheck.c          \
   vtls/keylog.c             \
@@ -61,7 +60,6 @@ LIB_VTLS_CFILES =           \
 
 LIB_VTLS_HFILES =           \
   vtls/bearssl.h            \
-  vtls/gskit.h              \
   vtls/gtls.h               \
   vtls/hostcheck.h          \
   vtls/keylog.h             \