hwpmc: remove stub pmd_switch_{in,out} methods

Most platforms (non-x86) don't require these methods and implement stub
versions. If we initialize the pmc_mdep structure to always point to the
generic versions, then we can purge the duplicate stubs.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39913