sched_ext: Remove redundant p->nr_cpus_allowed checker

select_rq_task() already checked that 'p->nr_cpus_allowed > 1',
'p->nr_cpus_allowed == 1' checker in scx_select_cpu_dfl() is redundant.

Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>