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.

(cherry picked from commit 22cdafe)

sys/sys/cdefs.h

@@ -278,15 +278,6 @@
 #if (defined(__cplusplus) && __cplusplus >= 201103L) || \
     __has_extension(cxx_static_assert)
 #define	_Static_assert(x, y)	static_assert(x, y)
-#elif __GNUC_PREREQ__(4,6) && !defined(__cplusplus)
-/* Nothing, gcc 4.6 and higher has _Static_assert built-in */
-#elif defined(__COUNTER__)
-#define	_Static_assert(x, y)	__Static_assert(x, __COUNTER__)
-#define	__Static_assert(x, y)	___Static_assert(x, y)
-#define	___Static_assert(x, y)	typedef char __assert_ ## y[(x) ? 1 : -1] \
-				__unused
-#else
-#define	_Static_assert(x, y)	struct __hack
 #endif
 #endif