posix-cpu-timers: Always clear head pointer on dequeue

The head pointer in struct cpu_timer is checked to be NULL in
posix_cpu_timer_del() when the delete raced with the exit cleanup. The
works correctly as long as the timer is actually dequeued via
posix_cpu_timers_exit*().

But if the timer was dequeued due to expiry the head pointer is still set
and triggers the warning.

In fact keeping the head pointer around after any dequeue is pointless as
is has no meaning at all after that.

Clear the head pointer always on dequeue and remove the unused requeue
function while at it.

Fixes: 60bda037

 ("posix-cpu-timers: Utilize timerqueue for storage")
Reported-by:  <syzbot+55acd54b57bb4b3840a4@syzkaller.appspotmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190905120539.707986830@linutronix.de