hwpmc: unused/diagused annotations in tsc class

These are preferred over casts to void. No functional change.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39914

(cherry picked from commit a35453b9c7644f11df83d0c7e73da488f521e56e)