ExtensionSystem: fix uninstalling from website

uninstall function was still referring to the old state and error
objects, which were removed by the refactoring.

https://bugzilla.gnome.org/show_bug.cgi?id=671134

js/ui/extensionSystem.js

@@ -103,8 +103,6 @@ function uninstallExtensionFromUUID(uuid) {
     _signals.emit('extension-state-changed', extension);
 
     delete ExtensionUtils.extensions[uuid];
-    delete extensionStateObjs[uuid];
-    delete errors[uuid];
 
     FileUtils.recursivelyDeleteDir(Gio.file_new_for_path(extension.path));