sched: kill unused scheduler decl.

I noticed this while making investigations into the tbench
regressions.  Please apply.

sched: Remove hrtick_resched() extern decl.

This function was removed by 31656519e132f6612584815f128c83976a9aaaef
("sched, x86: clean up hrtick implementation").

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>