x86 atomics: Remove unused WANT_FUNCTIONS

This macro has not been in use since commit "inline atomics and allow tied
modules to inline locks" (r335873, f4b3640475cec929).

Reviewed by: markj, kib, emaste, imp
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48061

(cherry picked from commit fa368cc86cebe7185b3a99d4f6083033da377eee)