[mips] Removed MipsCC::fixedArgFn(). NFC

Summary:
There is one remaining trace of it in MipsCC::analyzeCallOperands() where
Mips16 might override the calling convention. This will moved into
tablegen-erated code later.

Depends on D5965

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5966