Some time ago (997a34bf8c) the backlog
controller was generalized to maintain some scheduling group. Back then
the group was the pair of seastar::scheduling_group and
seastar::io_priority_class. Now the latter is gone, so the controller's
notion of what sched group is can be relaxed.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#14266