Remove unused parameter from showSnackback (#1192)

src/Screen.js

@@ -121,7 +121,7 @@ class Navigator {
   }
 
   showSnackbar(params = {}) {
-    return platformSpecific.showSnackbar(this, params);
+    return platformSpecific.showSnackbar(params);
   }
 
   dismissSnackbar() {

src/deprecated/platformSpecificDeprecated.android.js

@@ -624,7 +624,7 @@ function addNavigationStyleParams(screen) {
   screen.navigatorStyle = Object.assign({}, Screen.navigatorStyle, screen.navigatorStyle);
 }
 
-function showSnackbar(navigator, params) {
+function showSnackbar(params) {
   const adapted = _.cloneDeep(params);
   if (adapted.backgroundColor) {
     adapted.backgroundColor = processColor(adapted.backgroundColor);