reactor: unregister metrics of queue on shard 0
reactor: unregister metrics of queue on shard 0

reactor::stop calls smp::cleanup_cpu for each of the smp queues, a call
that cleans shard 0 was missing from outside of the loop (that only
handles shard > 0)

Message-Id: <6fb2ad5fd40e99ea5aa0cf9cc01d4cc971db5aeb@scylladb.com>

avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar