[AArch64][MC] Merge FeaturePMU into FeaturePerfMon

FeaturePMU was created in AArch64 to accommodate one missing system
register, PMMIR_EL1, in commit ffcd7698.

However, the Performance Monitors extension already had a target
feature, which is called FeaturePerfMon. Therefore, FeaturePMU is
redundant.

This patch removes FeaturePMU and merges its contents into
FeaturePerfMon.

Reviewed By: dnsampaio

Differential Revision: https://reviews.llvm.org/D109246