MIPS: Remove unused StubType's, freeing 2 bits in Code objects.

Port r17867 (a305bd3)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/60763005

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17885 ce2b1a6d-e550-0410-aec6-3dcde31c8c00