sched_ext: Always use SMP versions in kernel/sched/ext_idle.h

Simplify the scheduler by making formerly SMP-only primitives and data
structures unconditional.

tj: Updated subject for clarity.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>