workspaces-view: Simplify handling of removed workspaces

Workspaces used to contain the desktop background, so when a
workspace was removed, we animated its actor to an off-screen
position before destroying it. As the background has been
removed a while ago, we can destroy the actor directly.

https://bugzilla.gnome.org/show_bug.cgi?id=645031

js/ui/workspace.js

@@ -641,16 +641,6 @@ Workspace.prototype = {
         return this._windows.length == 0;
     },
 
-    /**
-     * setReactive:
-     * @reactive: %true iff the workspace should be reactive
-     *
-     * Set the workspace (desktop) reactive
-     **/
-    setReactive: function(reactive) {
-        this.actor.reactive = reactive;
-    },
-
     // Only use this for n <= 20 say
     _factorial: function(n) {
         let result = 1;