cdefs.h: Don't define fallback for _Static_assert

Remove pre 4.6 code to define _Static_assert in terms of _COUNTER. We
no longer need to support compilers this old (in fact support for all
pre gcc 10 compilers has been removed in -current). This is a partial
MFC of that work because removing this fixes a bug that's oft reported
with -pedantic-errors and C++98 compilations.

PR: 280382, 276738
Sponsored by: Netflix

This is a direct commit to stable/14.