libkern.h: remove HAVE_INLINE_* macros

The final consumer of this was OpenZFS, fixed in ffaea0831973 (thanks
mav@). That change has been present in all active OpenZFS release
branches for at least 6 months. These can finally be retired.

Reviewed by: mav
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55201