MIPS: Simplify string copy in SubStringStub.

Port r21756 (3d58e58)

Original commit message:
The optimizations are unnecessary since the maximum
string length they operate on is currently 12.

BUG=352155
LOG=N
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/326413002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21773 ce2b1a6d-e550-0410-aec6-3dcde31c8c00