MIPS: port Remove variable rewrites and the unneccesary Slot class.

Port r9162 (2215df8).

BUG=
TEST=

Review URL: http://codereview.chromium.org/7847015
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9187 ce2b1a6d-e550-0410-aec6-3dcde31c8c00