[mips] EABI CodeGen is completely untested and seems to have bitrotted. Remove it.

Summary:
There are no tests*, no EABI buildbots, and simple test cases do not work.

* There is a single MIPS16 test using a mips*-gnueabi triple but this test
  doesn't test EABI and the triple doesn't cause EABI to be used.

Reviewers: sdardis

Subscribers: tberghammer, danalbert, srhines, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20906

llvm-svn: 271658