MIPS: Remove dead code for inline string hashing.

Port r23689 (e8762c3)

Original commit message:
This has become obsolete since hydrogenized StringAdd.

BUG=
R=dusan.milosavljevic@imgtec.com

Review URL: https://codereview.chromium.org/542663002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23701 ce2b1a6d-e550-0410-aec6-3dcde31c8c00