Remove status message check from node interop client

The node interop client was checking that the status message for
the test unimplemented_method was an empty string. This behavior
is not guaranteed, and thus the check should be removed.

interop/interop_client.js

@@ -380,7 +380,6 @@ function unimplementedService(client, done) {
   client.unimplementedCall({}, function(err, resp) {
     assert(err);
     assert.strictEqual(err.code, grpc.status.UNIMPLEMENTED);
-    assert(!err.message);
     done();
   });
 }