multi: remove ErrDoubleSpend check for PublishTransaction

In this commit, we address a lingering issue within some subsystems that
are responsible for broadcasting transactions. Previously,
ErrDoubleSpend indicated that a transaction was already included in the
mempool/chain. This error was then modified to actually be returned for
conflicting transactions, but its callers were not modified accordingly.
This would lead to conflicting transactions to be interpreted as valid,
when they shouldn't be.

watchtower/lookout/punisher.go

@@ -2,7 +2,6 @@ package lookout
 
 import (
 	"github.com/btcsuite/btcd/wire"
-	"github.com/lightningnetwork/lnd/lnwallet"
 )
 
 // PunisherConfig houses the resources required by the Punisher.
@@ -44,7 +43,7 @@ func (p *BreachPunisher) Punish(desc *JusticeDescriptor, quit <-chan struct{}) e
 		desc.SessionInfo.ID, justiceTxn.TxHash())
 
 	err = p.cfg.PublishTx(justiceTxn)
-	if err != nil && err != lnwallet.ErrDoubleSpend {
+	if err != nil {
 		log.Errorf("Unable to publish justice txn for client=%s"+
 			"with breach-txid=%s: %v",
 			desc.SessionInfo.ID, desc.BreachedCommitTx.TxHash(), err)