Use meta_window_is_skip_taskbar() directly

Rather than going through ShellWindowTracker, we can just clean up
the code to use the underlying MetaWindow property directly.

https://bugzilla.gnome.org/show_bug.cgi?id=723308

js/ui/workspaceThumbnail.js

@@ -507,8 +507,7 @@ const WorkspaceThumbnail = new Lang.Class({
 
     // Tests if @win should be shown in the Overview
     _isOverviewWindow : function (win) {
-        let tracker = Shell.WindowTracker.get_default();
-        return tracker.is_window_interesting(win.get_meta_window()) &&
+        return !win.get_meta_window().skip_taskbar &&
                win.get_meta_window().showing_on_its_workspace();
     },