[TargetRegisterInfo] Remove temporary hook enableMultipleCopyHints()

Finally all targets are enabling multiple regalloc hints, so the hook to
disable this can now be removed.

NFC.

Review: Simon Pilgrim
https://reviews.llvm.org/D52316

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343851 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed