sched_clock: cleanups

- remove superfluous checks in __update_sched_clock()
- skip sched_clock_tick() for sched_clock_stable
- reinstate the simple !HAVE_UNSTABLE_SCHED_CLOCK code to please the bloatwatch

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>