Comment 2 for bug 1463680

Oh right, this is because update_mirrors.py only adds mirrors and doesn't remove them.

 13 d = feedparser.parse("https://launchpad.net/ubuntu/+archivemirrors-rss")
 14
 15 #import pprint
 16 #pp = pprint.PrettyPrinter(indent=4)
 17 #pp.pprint(d)
 18
 19 with open(sys.argv[1], "a") as outfile:
 20 for entry in d.entries:
 21 for link in entry.links:
 22 if link.href not in current_mirrors:
 23 outfile.write(link.href + "\n")

Having said that I'm not certain what ubuntu-release-upgrader would do if your mirror wasn't in that list.