loginDialog: Hide session menu button when showing the user list

In the past, the session menu button was part of the auth prompt widgetry,
so we didn't have to manually hide it when showing the user list. However,
now it is part of the login screen itself.

Hide the session menu button when the user list is shown.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/999

js/gdm/loginDialog.js

@@ -1139,6 +1139,7 @@ var LoginDialog = GObject.registerClass({
         this._authPrompt.hide();
         this._hideBannerView();
         this._sessionMenuButton.close();
+        this._sessionMenuButton.hide();
         this._setUserListExpanded(true);
         this._notListedButton.show();
         this._userList.grab_key_focus();