amd64: Remove PMAP_INLINE

With clang it expands to "inline"; clang in practice may inline
externally visible functions even without the hint. So just remove the
hints and let the compiler decide.

No functional change intended. pmap.o is identical before and after
this patch.

Reviewed by: alc
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42446