Remove storage change-event listener from Safari

Earlier, a technique was used to open the extension's Options page when
the user clicked a checkbox input at Safari's extension settings. The method was removed because:
- the Options page can be opened via the extension's toolbar button (which
  cannot be disabled in Safari, so it will be there all the time);
- involved more clicks than opening from the toolbar button;
- the string beside the checkbox couldn't be localized.

src/js/vapi-background.js

@@ -263,12 +263,6 @@ if (window.chrome) {
 } else if (window.safari) {
     vAPI.safari = true;
 
-    safari.extension.settings.addEventListener('change', function(e) {
-        if (e.key === 'open_prefs') {
-            vAPI.tabs.open({url: 'dashboard.html', active: true});
-        }
-    }, false);
-
     vAPI.storage = {
         _storage: safari.extension.settings,
         QUOTA_BYTES: 52428800, // copied from Info.plist