Don't send ActiveLeaves from leaves in db on startup in Overseer (#6727)

* Don't send `ActiveLeaves` from leaves in db on startup in Overseer. Wait for fresh leaves instead.

* Don't pass initial set of leaves to Overseer

* Fix compilation error in subsystem-test-helpers

polkadot/node/subsystem-test-helpers/src/lib.rs

@@ -457,7 +457,6 @@ mod tests {
 			dummy_overseer_builder(spawner.clone(), AlwaysSupportsParachains, None)
 				.unwrap()
 				.replace_collator_protocol(|_| ForwardSubsystem(tx))
-				.leaves(vec![])
 				.build()
 				.unwrap();