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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243638 91177308-0d34-0410-b5e6-96231b3b80d8