hwpmc: remove static POWER8 definitions

After b48a2770d48b, static POWER8 definitions became unnecessary,
as all of them (and much more) are already present in libpmc's
PMU events.

Submitted by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br> (initial version)
Reviewed by: kbowling, mhorne
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31334