[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339125 91177308-0d34-0410-b5e6-96231b3b80d8