multi: use txid instead of tx in BreachRetribution

This commit replaces the field `BreachTransaction` with `BreachTxHash`
as there's no need to pass that large raw transaction.

watchtower/wtclient/backup_task.go

@@ -346,7 +346,7 @@ func (t *backupTask) craftSessionPayload(
 		}
 	}
 
-	breachTxID := t.breachInfo.BreachTransaction.TxHash()
+	breachTxID := t.breachInfo.BreachTxHash
 
 	// Compute the breach key as SHA256(txid).
 	hint, key := blob.NewBreachHintAndKeyFromHash(&breachTxID)