[mips] Remove redundant 'let Predicates = [HasStdEnc]' statements

Summary:
The MipsPat class already initializes Predicates to [HasStdEnc].

No functional change (confirmed by diffing tablegen-erated files before and
after)

Differential Revision: http://reviews.llvm.org/D3546

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207548 91177308-0d34-0410-b5e6-96231b3b80d8