MIPS: Remove the obsolete Code::IsPregenerated flag.

Port r18172 (cd823192)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/99323003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18208 ce2b1a6d-e550-0410-aec6-3dcde31c8c00