Remove preemption thread and API

BUG=v8:3004
R=svenpanne@chromium.org, yangguo@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/62283010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17967 ce2b1a6d-e550-0410-aec6-3dcde31c8c00