multi: remove Litecoin config options

This commit removes the `Litecoin`, `LtcMode` and `LitecoindMode`
members from the main LND `Config` struct. Since only the bitcoin
blockchain is now supported, this commit also deprecates the
`cfg.Bitcoin.Active` config option.

rpcserver.go

@@ -5570,9 +5570,6 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 	invoice *lnrpc.Invoice) (*lnrpc.AddInvoiceResponse, error) {
 
 	defaultDelta := r.cfg.Bitcoin.TimeLockDelta
-	if r.cfg.registeredChains.PrimaryChain() == chainreg.LitecoinChain {
-		defaultDelta = r.cfg.Litecoin.TimeLockDelta
-	}
 
 	addInvoiceCfg := &invoicesrpc.AddInvoiceConfig{
 		AddInvoice:        r.server.invoices.AddInvoice,