libpmc: eliminate pmc_pmu_stat_mode()

There is a single consumer, the pmc utility, that clearly has knowledge
of which counters it is expecting. Remove this function and have it
use common counter aliases instead.

Reviewed by: gnn
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30528

(cherry picked from commit ec66cc955b629e614cf493bf168048de033f6a2c)