libpmc: remove PMC_MDEP_TABLE logic

This logic was added for handling some of the complicated relationships
between events and x86 CPU models. Since that logic has been mostly
removed from libpmc(3) in favor of pmu-events, this no longer serves
much of a purpose. Mapping CPU types to event tables is already handled
by the switch statement in pmc_init().

Reviewed by: ray, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30196