sched/deadline: Remove unnecessary comment in dl_add_task_root_domain()

sched/deadline: Remove unnecessary comment in dl_add_task_root_domain()

The comments above dl_get_task_effective_cpus() and
dl_add_task_root_domain() already explain how to fetch a valid
root domain and protect against races. There's no need to repeat
this inside dl_add_task_root_domain(). Remove the redundant comment
to keep the code clean.

No functional change.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://patch.msgid.link/20251125032630.8746-2-piliu@redhat.com