Remove NetworkSpecialization::on_event (#3976)

substrate/core/network/src/protocol/specialization.rs

@@ -42,7 +42,8 @@ pub trait NetworkSpecialization<B: BlockT>: Send + Sync + 'static {
 	);
 
 	/// Called when a network-specific event arrives.
-	fn on_event(&mut self, event: Event);
+	#[deprecated(note = "This method is never called; please use `with_dht_event_tx` when building the service")]
+	fn on_event(&mut self, event: Event) {}
 
 	/// Called on abort.
 	#[deprecated(note = "This method is never called; aborting corresponds to dropping the object")]