overview: Remove unneeded check if primary monitor exists

This check is no longer needed because we're using the stage size
instead of the monitor size here.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/374

js/ui/overview.js

@@ -387,9 +387,6 @@ var Overview = class {
         // when it is next shown.
         this.hide();
 
-        if (!Main.layoutManager.primaryMonitor)
-            return;
-
         this._coverPane.set_position(0, 0);
         this._coverPane.set_size(global.screen_width, global.screen_height);