extensionSystem: Remove the two-step initialize

Initially, extensions were loaded after they shell had fully created
the session and all objects, but this didn't allow extensions easy
ways to monkey patch prototypes, as most functions had already been
bound. Remove the historical vestigal function, and just merge the
two together.

https://bugzilla.gnome.org/show_bug.cgi?id=677586

js/ui/main.js

@@ -221,7 +221,6 @@ function start() {
                                                 false, -1, 1);
 
     if (sessionMode.allowExtensions) {
-        ExtensionSystem.init();
         ExtensionDownloader.init();
         ExtensionSystem.loadExtensions();
     }