(bug 6387) Introduced new setting $wgCategoryPrefixedDefaultSortkey which

(bug 6387) Introduced new setting $wgCategoryPrefixedDefaultSortkey which
allows having the unprefixed page title as the default category sortkey.

Although creating sane defaults should always be preferred over introducing
new config options, we cannot just remove the old behaviour here, as some
peoply might still rely on it. However, the sortkey {{PAGENAME}} is already
widely used for circumventing the current behaviour.