hwpmc: drop vestigial IAP event definitions

These are maintained elsewhere. No functional change.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39900

(cherry picked from commit 1be8ef26794dcf93e2f4851474808dc3b34dbcbd)