Remove mips build.

For now this leaves the ability to cross-unwind a mips process, but we
should probably clean that up too. We need to remove the build remnants so
that we can clean up the build system itself (otherwise it sees us talking
about an architecture it doesn't know about, and assumes that something's
wrong).

Test: treehugger
Change-Id: I2862c630cec95dbdd474e34c3568d0e1a6d44b16