Drop unused rdtsc instruction.

We do not use rdtsc anywhere and we won't ever use that in the
future, as it is totally unusable with multicore and out of
order execution.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/23112016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16268 ce2b1a6d-e550-0410-aec6-3dcde31c8c00