sched/idle,stop: Remove .get_rr_interval from sched_class

The idle task and stop task sched_classes return 0 in this function.

The single call site in sched_rr_get_interval() calls
p->sched_class->get_rr_interval() only conditional in case it is
defined. Otherwise time_slice=0 will be used.

The deadline sched class does not define it. Commit a57beec5
("sched: Make sched_class::get_rr_interval() optional") introduced
the default time-slice=0 for sched classes which do not provide this
function.

So .get_rr_interval for idle and stop sched_class can be removed to
shrink the code a little.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200603080304.16548-4-dietmar.eggemann@arm.com