sched/core: Remove the pointless BUG_ON(!task) from wake_up_q()

container_of() can never return NULL - so don't check for it pointlessly.

[ mingo: Twiddled the changelog. ]

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210510161522.GA32644@redhat.com