Do not returning external title in ApiPageSet::getRedirectTargets

Having a page with a redirect to "en:Main Page" will be added as "Main
Page" to the output, that is wrong

http://test.wikipedia.org/w/api.php?action=query&titles=Redirect%20to%20interwiki%20page&redirects

Change-Id: I7b59f592c1293dbb96f4c4279201bf04fcfea29d