Remove __GNUC__ / _MSC_VER games from win32/config_H.*

Remove __GNUC__ / _MSC_VER games from win32/config_H.*

The code was added by 465b7da985, based on changes in ActivePerl 816, but
it's not working correctly because the current build process winds up with
a config.h in lib/CORE which doesn't contain that code anyway.

(The appropriate canned config_H.* file is copied to config.h in order to
build miniperl.exe, but then miniperl.exe is used to run config_h.PL to
generate the real config.h from config_h.SH and config.sh. The real
config.h is then used to build perl.exe and is what gets copied into
lib/CORE for later use when building extensions, but it doesn't contain
the support for other compilers because that isn't in config_h.SH...)

Removing the code won't affect ActivePerl since it has not been getting
installed anyway, and Jan Dubois has confirmed that it is possible to
build a Perl extension using e.g. Off_t with MinGW even with the VC
config.h header file. Furthermore, anyone building perl from source
themselves will presumably be able to use the same compiler to build
extensions as they used to build perl itself anyway, so there is no
obvious need for this in the core perl distro.

Therefore, removing it rather than fixing it is better for simplicity.