swarm/: Provide additional default impls on NetworkBehaviour (#2150)

Not all implementations of `NetworkBehaviour` need all callbacks.
We've have been adding new callbacks with default implementations
for a while now. There is no reason the initial ones cannot also
be defaulted, thus making it easier create new implementations.

Co-authored-by: Max Inden <mail@max-inden.de>

protocols/gossipsub/src/behaviour.rs

@@ -2804,10 +2804,6 @@ where
         )
     }
 
-    fn addresses_of_peer(&mut self, _: &PeerId) -> Vec<Multiaddr> {
-        Vec::new()
-    }
-
     fn inject_connected(&mut self, peer_id: &PeerId) {
         // Ignore connections from blacklisted peers.
         if self.blacklisted_peers.contains(peer_id) {