multi: remove breach tx arg from NewBreachRetribution args

This commit removes the breach transaction from the
arguments passed to NewBreachRetribution. We already
keep all prior remote commitments on disk in the
commitment log, and load that transaction from disk
inside the method. In practice, the one loaded from
disk will be the same one that is passed in by the
caller, so there should be no change in behavior
as we've already derived the appropriate state number.

This changes makes integration with the watchtower
client simpler, since we no longer need to acquire
the breaching commitment transaction to be able to
construct the BreachRetribution. This simplifies
not only the logic surrounding transient backsups,
but also on startup (and later, retroactively
backing up historic updates).

contractcourt/chain_watcher.go

@@ -725,18 +725,14 @@ func (c *chainWatcher) dispatchContractBreach(spendEvent *chainntnfs.SpendDetail
 		return fmt.Errorf("unable to mark channel as borked: %v", err)
 	}
 
-	var (
-		commitTxBroadcast = spendEvent.SpendingTx
-		spendHeight       = uint32(spendEvent.SpendingHeight)
-	)
+	spendHeight := uint32(spendEvent.SpendingHeight)
 
 	// Create a new reach retribution struct which contains all the data
 	// needed to swiftly bring the cheating peer to justice.
 	//
 	// TODO(roasbeef): move to same package
 	retribution, err := lnwallet.NewBreachRetribution(
-		c.cfg.chanState, broadcastStateNum, commitTxBroadcast,
-		spendHeight,
+		c.cfg.chanState, broadcastStateNum, spendHeight,
 	)
 	if err != nil {
 		return fmt.Errorf("unable to create breach retribution: %v", err)