MC: Remove MCSubtargetInfo::InitCPUSched()

Remove all calls to `MCSubtargetInfo::InitCPUSched()` and merge its body
into the only relevant caller, `MCSubtargetInfo::InitMCProcessorInfo()`.
We were only calling the former after explicitly calling the latter with
the same CPU; it's confusing to have both methods exposed.

Besides a minor (surely unmeasurable) speedup in ARM and X86 from
avoiding running the logic twice, no functionality change.

llvm-svn: 241956