rpc: allow wumbo invoices

In this commit, we remove the restriction surrounding the largest
invoices that we'll allow a user to create. After #3967 has landed,
users will be able to send in _aggregate_ a payment larger than the
current max HTLC size limit in the network. As a result, we can just
treat that value as the system's MTU, and allow users to request
payments it multiples of that MTU value.

A follow up to this PR at a later time will also allow wumbo _channels_.
However, that requires us to tweak the way we scale CSV values, as post
wumbo, there is no true channel size limit, only the
_local_ limit of a given node. We also need to implement a way for nodes
to signal to other nodes their accepted max channel size.

rpcserver.go

@@ -4305,7 +4305,6 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 		IsChannelActive:   r.server.htlcSwitch.HasActiveLink,
 		ChainParams:       activeNetParams.Params,
 		NodeSigner:        r.server.nodeSigner,
-		MaxPaymentMSat:    MaxPaymentMSat,
 		DefaultCLTVExpiry: defaultDelta,
 		ChanDB:            r.server.chanDB,
 		GenInvoiceFeatures: func() *lnwire.FeatureVector {