Simplify string copy in SubStringStub.

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

R=bmeurer@chromium.org
BUG=352155
LOG=N

Review URL: https://codereview.chromium.org/326943002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21756 ce2b1a6d-e550-0410-aec6-3dcde31c8c00