[mips] Removed the remainder of MipsCC. NFC.

Summary:
One of the calls to AllocateStack (the one in LowerCall) doesn't look like
it should be there but it was there before and removing it breaks the
frame size calculation.

Reviewers: vmedic, theraven

Reviewed By: theraven

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6116