net/sched: remove qdisc_root_lock() helper

the last caller has been removed with commit 96f5e66e ("mac80211: fix
aggregation for hardware with ampdu queues"), so it's safe to remove this
function.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/703d549e3088367651d92a059743f1be848d74b7.1658133689.git.dcaratti@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>