Drop unused v8threads include from platform files

BUG=none
R=marja@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/323303002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21788 ce2b1a6d-e550-0410-aec6-3dcde31c8c00