Remove outpoint_to_peer map in ChannelManager

Now that we require `ChannelMonitor`s to track the channel's
counterparty node ID, we can remove the `outpoint_to_peer` map that was
used throughout `MonitorEvent` handling. This is a big win for a
post-splicing future, as we'll no longer have to bother updating this
map when a splice is proposed.

Some existing tests providing coverage have been removed as a result.

Author: wpaulino

lightning/src/ln/async_signer_tests.rs

@@ -18,7 +18,6 @@ use bitcoin::transaction::Version;
 
 use crate::chain::channelmonitor::LATENCY_GRACE_PERIOD_BLOCKS;
 use crate::chain::ChannelMonitorUpdateStatus;
-use crate::chain::transaction::OutPoint;
 use crate::events::bump_transaction::WalletSource;
 use crate::events::{ClosureReason, Event, MessageSendEvent, MessageSendEventsProvider};
 use crate::ln::chan_utils::ClosingTransaction;
@@ -1091,9 +1090,4 @@ fn do_test_closing_signed(extra_closing_signed: bool, reconnect: bool) {
 	assert!(nodes[1].node.list_channels().is_empty());
 	check_closed_event!(nodes[0], 1, ClosureReason::LocallyInitiatedCooperativeClosure, [nodes[1].node.get_our_node_id()], 100000);
 	check_closed_event!(nodes[1], 1, ClosureReason::CounterpartyInitiatedCooperativeClosure, [nodes[0].node.get_our_node_id()], 100000);
-
-	// Check that our maps have been updated after async signing channel closure.
-	let funding_outpoint = OutPoint { txid: funding_tx.compute_txid(), index: 0 };
-	assert!(nodes[0].node().outpoint_to_peer.lock().unwrap().get(&funding_outpoint).is_none());
-	assert!(nodes[1].node().outpoint_to_peer.lock().unwrap().get(&funding_outpoint).is_none());
 }