[mips] Kill 'support' for untested EABI.
There are no llvm backend tests* for EABI and no EABI buildbots. There were only
three clang tests, all of which checked that -mabi=eabi was passed to the
*There is a single backend test that specifies EABI but it actually tests MIPS16.
Subscribers: emaste, sdardis, atanasyan, cfe-commits
Differential Revision: http://reviews.llvm.org/D20679