hwpmc: don't validate capabilities in allocation method

These checks were inconsistently applied across the various hwpmc
classes. The condition is already checked by the generic code in
hwpmc_mod.c, so remove them.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31388

(cherry picked from commit 0e78510b75497d183ab1aafbd99ff1031798bd84)