CLEANUP: task: remove the unused task_unlink_rq()

CLEANUP: task: remove the unused task_unlink_rq()

This function stopped being used before 2.4 because either the task is
dequeued by the scheduler itself and it knows where to find it, or it's
killed by any thread, and task_kill() must be used for this as only this
one is safe.

It's difficult to say whether task_unlink_rq() is still safe, but once
the lock moves to a thread declared in the task itself, it will be even
more difficult to keep it safe.

Let's just remove it now before someone reuses it and causes trouble.