[Tablegen] In TargetSchedule.td: Remove unused argument `pfmCounters` from ProcResourceUnits.

PFM counters don't need to be passed in input to the definition of
ProcResourceUnits. class PfmIssueCounter (see r329675) is used to map resources
to PFM counter(s).

Differential Revision: https://reviews.llvm.org/D50333

llvm-svn: 339125