libpmc: eliminate pmc_pmu_stat_mode()

[mhorne]

Summary

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.

No functional change intended.

[mhorne]

usr.sbin/pmc/cmd_pmc_stat.c

114

Since it is not obvious from the diff, these aliases refer to the same counters as the names being removed above (on intel at least). Therefore, later calls to pmc_allocate() in this file will behave the same.