lnwallet: no need to grab mutex in ProcessChanSyncMsg as is init method

In this method we fix an existing deadlock within the unit tests when
running with the race condition detector on. We don’t need to grab the
mutex within the ProcessChanSyncMsg method as this should be the very
first method called when initializing the channel if a channel state
sync is needed.

lnwallet/channel.go

@@ -3000,8 +3000,6 @@ func (lc *LightningChannel) SignNextCommitment() (*btcec.Signature, []*btcec.Sig
 //  * RevokeAndAck: if we sent a revocation message that they claim to have
 //    not received
 func (lc *LightningChannel) ProcessChanSyncMsg(msg *lnwire.ChannelReestablish) ([]lnwire.Message, error) {
-	lc.Lock()
-	defer lc.Unlock()
 
 	// We owe them a commitment if they have an un-acked commitment and the
 	// tip of their chain (from our Pov) is equal to what they think their