pmc_events.h: update event list comment

This comment is no longer in sync with the contents of __PMC_EVENTS().
Update to reflect the removal of various Intel event definitions from
this list; these event definitions now come from Linux and live in
lib/libpmc/pmu-events/.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39902

(cherry picked from commit 6bdf067890ed1d1515ad0dbd105848bb11711210)