reactor: don't register I/O pollers in shards without an I/O Queue
reactor: don't register I/O pollers in shards without an I/O Queue

Shards that can't issue I/O, have nothing to poll when it comes to I/O.
Simplify our code by not even registering them pollers in this case.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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