Don't sort the podcast model in-place

This is already done in all the views, so it's
unnecessary to sort the "backend" list, too.

src/gpodder/gtkui/main.py

@@ -2196,7 +2196,6 @@ class gPodder(BuilderWidget, dbus.service.Object):
                 url = None
 
             # Update the list of subscribed podcasts
-            self.channels.sort(key=Model.podcast_sort_key)
             self.update_podcast_list_model(select_url=url)
 
             # Offer to download new episodes

src/gpodder/model.py

@@ -1278,8 +1278,7 @@ class Model(object):
             return self.PodcastClass.create_from_dict(dct, self)
 
         if self.children is None:
-            self.children = sorted(self.db.load_podcasts(podcast_factory),
-                    key=self.PodcastClass.sort_key)
+            self.children = self.db.load_podcasts(podcast_factory)
         return self.children
 
     def load_podcast(self, url, create=True, authentication_tokens=None,