blk-mq: Drop explicit timeout sync in hotplug

After commit 287922eb ("block: defer timeouts to a workqueue"),
deleting the timeout work after freezing the queue shouldn't be
necessary, since the synchronization is already enforced by the
acquisition of a q_usage_counter reference in blk_mq_timeout_work.

Signed-off-by: Gabriel Krisman Bertazi <krisman@linux.vnet.ibm.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>