kernel/sched: Remove dl_boosted flag comment

since commit 2279f540 ("sched/deadline: Fix priority
inheritance with multiple scheduling classes"), we should not
keep it here.

Signed-off-by: Hui Su <suhui_kernel@163.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Link: https://lore.kernel.org/r/20220107095254.GA49258@localhost.localdomain