fix(rn,conference) remove no longer needed margin

We are using a safe area view now.

react/features/conference/components/native/styles.js

@@ -135,11 +135,7 @@ export default {
         left: 0,
         position: 'absolute',
         right: 0,
-
-        // Both on Android and iOS there is the status bar which may be visible.
-        // On iPhone X there is the notch. In the two cases BoxModel.margin is
-        // not enough.
-        top: BoxModel.margin * 3
+        top: 0
     },
 
     insecureRoomNameLabel: {