sched: No PREEMPT_RT=y for all{yes,mod}config

While PREEMPT_RT is undoubtedly totally awesome, it does not, at this
time, make sense to have all{yes,mod}config select it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 35772d62 ("sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>