sched_ext: Open-code task_linked_on_dsq()

sched_ext: Open-code task_linked_on_dsq()

task_linked_on_dsq() exists as a helper because it used to test both the
rbtree and list nodes. It now only tests the list node and the list node
will soon be used for something else too. The helper doesn't improve
anything materially and the naming will become confusing. Open-code the list
node testing and remove task_linked_on_dsq()

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>