[mips][FastISel] Remove hidden mips-fast-isel option.

Summary:
This hidden option would disable code generation through FastISel by
default. It was removed from the available options and from the
Fast-ISel tests that required it in order to run the tests.

Reviewers: dsanders

Subscribers: qcolombet, llvm-commits

Differential Revision: http://reviews.llvm.org/D11610

llvm-svn: 243638