Some fixes the compiler pointed out to me (#4482)

* Fix bug in availability-distribution.

Old fetches would not get cancelled as intended.

* Fix a few more warnings.

polkadot/node/network/availability-distribution/src/requester/fetch_task/mod.rs

@@ -203,7 +203,9 @@ impl FetchTask {
 	/// Remove leaves and cancel the task, if it was the last one and the task has still been
 	/// fetching.
 	pub fn remove_leaves(&mut self, leaves: &HashSet<Hash>) {
-		self.live_in.difference(leaves);
+		for leaf in leaves {
+			self.live_in.remove(leaf);
+		}
 		if self.live_in.is_empty() && !self.is_finished() {
 			self.state = FetchedState::Canceled
 		}