[mips] Remove MipsCC::getRegVT(). NFC

Summary: It's no longer used.

Reviewers: vmedic, theraven

Reviewed By: theraven

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6112

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221519 91177308-0d34-0410-b5e6-96231b3b80d8