Remove media patch, put usnLim in server code

Since dae/anki@d6874de8, usnLim always returns -1 regardless of the
`server` attribute.

Reference: d6874de8c8

ankisyncd/sync_app.py

@@ -91,6 +91,9 @@ class SyncCollectionHandler(anki.sync.Syncer):
             'cont': True,
         }
 
+    def usnLim(self):
+        return "usn >= %d" % self.minUsn
+
 class SyncMediaHandler(anki.sync.MediaSyncer):
     operations = ['begin', 'mediaChanges', 'mediaSanity', 'uploadChanges', 'downloadFiles']