cdefs.h: Remove NO_ANSI_KEYWORDS

Remove very old, deletion of ANSI keywords. Once upon a time, it was
common to want to do this. Then we only did it for non gcc compilers,
then all compilers claimed to be gcc, so we never did it. These days,
it's about useless to keep around since there's no compilers we can test
it with. It's an odd little artifact. Also, the advice in the comment is
wrong: if you define NO_ANSI_KEYWORDS, you can't have progams that use
const, etc as identifiers because they will be deleted. Further, our
header files have the 'bare' ANSI forms of these, so it's a catch 22:
You'd need them deleted for .h files, but not for the code. It's such a
niche case these days that it's best to just remove it.

Sponsored by: Netflix
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D45861