lnrpc: remove reference to GOPATH

As go modules allow us to build outside of the GOPATH, we shouldn't look for .proto imports there anymore.

lnrpc/walletrpc/walletkit.proto

@@ -1,9 +1,11 @@
 syntax = "proto3";
 
-import "github.com/lightningnetwork/lnd/lnrpc/signrpc/signer.proto";
+import "signrpc/signer.proto";
 
 package walletrpc;
 
+option go_package = "github.com/lightningnetwork/lnd/lnrpc/walletrpc";
+
 message KeyReq {
     /**
     Is the key finger print of the root pubkey that this request is targeting.