MIPS: Fix build after r17654.

FullCodeGenerator::EmitMathFloor(CallRuntime* expr) function is not used anywhere and breaks the build.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/69293010

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17674 ce2b1a6d-e550-0410-aec6-3dcde31c8c00