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.

lnrpc/routerrpc/router_backend.go

@@ -698,18 +698,6 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 		payIntent.DestFeatures = features
 	}
 
-	// Currently, within the bootstrap phase of the network, we limit the
-	// largest payment size allotted to (2^32) - 1 mSAT or 4.29 million
-	// satoshis.
-	if payIntent.Amount > r.MaxPaymentMSat {
-		// In this case, we'll send an error to the caller, but
-		// continue our loop for the next payment.
-		return payIntent, fmt.Errorf("payment of %v is too large, "+
-			"max payment allowed is %v", payIntent.Amount,
-			r.MaxPaymentMSat)
-
-	}
-
 	// Check for disallowed payments to self.
 	if !rpcPayReq.AllowSelfPayment && payIntent.Target == r.SelfNode {
 		return nil, errors.New("self-payments not allowed")