pmc(3): remove Pentium-related man pages and references
Support for Pentium events was removed completely in e92a1350b50e.
Don't bump .Dd where we are just removing xrefs.
Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31423
(cherry picked from commit d78896e46f1d7744155919476c012400321d0dab)