sched: revert recent sync wakeup changes

Intel reported a 10% regression (mysql+sysbench) on a 16-way machine
with these patches:

  1596e297: sched: symmetric sync vs avg_overlap
  d942fb6c

: sched: fix sync wakeups

Revert them.

Reported-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Bisected-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>