libpmc: remove pe->alias

It has never been a part of upstream's struct pmu_event. The jevents
utility will not fill this field, so remove it.

Reviewed by: gnn
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30530

(cherry picked from commit 0c915023dbb7000cd30bb768eb84f6dc757adcc5)