Revert "Dispose Isolate at end of cctest."

Things work locally, only the waterfall is unhappy for some arcane reason.
Investigating...

TBR=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/133193006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19049 ce2b1a6d-e550-0410-aec6-3dcde31c8c00