Unbreak Special:ExtensionDistributor

$wgExtDistArchiveAPI was removed a while back, I never
removed the usage here apparently.

Bug: 69644
Change-Id: I39778d7fc642952172a67b895c3c546dd79af4f8
(cherry picked from commit 3116e09b4360d0c463da90f3f5526546c1ae7ae3)