hwpmc_amd: kill off k7 support bits

This is i386-only, and has been effectively disabled since 2018 when the
companion support was removed from libpmc (e92a1350b50e). Remove the
kernel support to simplify the AMD class.

Reviewed by: jkoshy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41274