This also deletes ObjectHashSet as it's no longer used.

BUG=v8:1793
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/225183009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20585 ce2b1a6d-e550-0410-aec6-3dcde31c8c00