lnrpc: deprecate unused pending closing channels field

The pending closing channels field was present to cover an edge case
where coperatively closed channels were closed before upgrade and
confirmed after the upgrade. This commit deprecates the field and adds
a warning log to cover these edge cases.

rpcserver.go

@@ -2725,22 +2725,15 @@ func (r *rpcServer) PendingChannels(ctx context.Context,
 
 		switch pendingClose.CloseType {
 
-		// If the channel was closed cooperatively, then we'll only
-		// need to tack on the closing txid.
-		// TODO(halseth): remove. After recent changes, a coop closed
-		// channel should never be in the "pending close" state.
-		// Keeping for now to let someone that upgraded in the middle
-		// of a close let their closing tx confirm.
+		// A coop closed channel should never be in the "pending close"
+		// state. If a node upgraded from an older lnd version in the
+		// middle of a their channel confirming, it will be in this
+		// state. We log a warning that the channel will not be included
+		// in the now deprecated pending close channels field.
 		case channeldb.CooperativeClose:
-			resp.PendingClosingChannels = append(
-				resp.PendingClosingChannels,
-				&lnrpc.PendingChannelsResponse_ClosedChannel{
-					Channel:     channel,
-					ClosingTxid: closeTXID,
-				},
-			)
-
-			resp.TotalLimboBalance += channel.LocalBalance
+			rpcsLog.Warn("channel %v cooperatively closed and "+
+				"in pending close state",
+				pendingClose.ChanPoint)
 
 		// If the channel was force closed, then we'll need to query
 		// the utxoNursery for additional information.