Don't display multiple language links to the same language

Don't display multiple language links to the same language

(bug 24502) Resolve the various issues with this accidental feature
by removing it. I think it could be done properly, along the lines of
my comment #5, but I don't think just changing the DB schema to make
langlinks non-unique is a good direction to take. A comment on
I4e1e08a3 from Daniel Kinzler indicates that duplicate language links
won't be possible with Wikidata anyway, so there's not much value in
I4e1e08a3 for WMF wikis.

Change-Id: Iba5f3f29e20f5119d4414b1e87ce5eee674701a8