(follow up r79706 to address CR comments) Simplify some of the logic in LinksUpdate.php

(follow up r79706 to address CR comments) Simplify some of the logic in LinksUpdate.php

Make it so that by default, the parser consdiers categories to have a sortkey of "" unless
specified otherwise. Before, the parser said the sortkey was the page name, and then in LinksUpdate
we reset the sortkey (prefix) to "" if it was the same as the page name.

This way, the parser uniformly outputs the sortkey prefix, instead of a mix between prefix or pagename.

It should be noted, this changes the output of api.php?action=parse for categories that do not have
a sortkey.