Delete mjsunit/string-oom-slow-* tests.

They are too slow, and there is no feasible way to speed them up.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/205553005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20186 ce2b1a6d-e550-0410-aec6-3dcde31c8c00