apps-menu: Remove more dead code

Commit 0a91893b02 removed dead code, namely a ::panel-box-changed signal
handler. Unfortunately, it forgot to remove the associated code that
disconnects the signal.

Signed-off-by: Alexander Rüedlinger <a.rueedlinger@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=785202

extensions/apps-menu/extension.js

@@ -494,7 +494,6 @@ const ApplicationsButton = new Lang.Class({
     _onDestroy: function() {
         Main.overview.disconnect(this._showingId);
         Main.overview.disconnect(this._hidingId);
-        Main.layoutManager.disconnect(this._panelBoxChangedId);
         appSys.disconnect(this._installedChangedId);
 
         Main.wm.setCustomKeybindingHandler('panel-main-menu',