Manual:$wgCategoryPrefixedDefaultSortkey

Details
Switch which determines if the namespace name should be included in (true, default) or removed from (false) the default category sortkey.
 * Set this to false in order to have a useful alphabetical default sort of non-ns0-pages in categories.
 * You have to run maintenance/refreshLinks.php after changing this.

Since 1.17 this is false always.