Revert: prctl: adds PR_SET_TIMERSLACK_PID for setting timer slack of an arbit…
…rary thread.

Second argument is similar to PR_SET_TIMERSLACK, if non-zero then the
slack is set to that value otherwise sets it to the default for the thread.

Takes PID of the thread as the third argument.

This allows power/performance management software to set timer slack for
other threads according to its policy for the thread (such as when the
thread is designated foreground vs. background activity) (reverted from commit 1463c5c8d5314093086480e53896a4e5ba6f6e28)