x86: Remove silly checks for <sys/cdefs.h>.

These headers #include <sys/cdefs.h> right after checking if it has
already been #included. The nested #include already existed when the
check for _SYS_CDEFS_H_ was added, so the check shouldn't have been
added in the first place.

PR: 263102 (exp-run)
Reported by: brooks
Reviewed by: brooks, imp, emaste
Differential Revision: https://reviews.freebsd.org/D34796

(cherry picked from commit 1c1bf5bd7c1e479a7889839b941f53e689aa2569)