MIPS: Remove unnecessary check in RegExpExecStub.

Port r22202 (cd07587)

BUG=v8:592
LOG=N
R=palfia@homejinni.com

Review URL: https://codereview.chromium.org/371503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22205 ce2b1a6d-e550-0410-aec6-3dcde31c8c00