hwpmc: remove pmd_pcpu_{init,fini} callbacks
These are unused on all platforms.
Reviewed by: jkoshy, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39912
(cherry picked from commit 772b2dc3abc5072743e7598d35e0a8ad621e7b0d)