Go to: "[MediaWiki]/includes/skins/Skin.php" and in line ~470-540 (do find exact by keywords $wgUseCategoryBrowser
, mw-normal-catlinks
, etc) change next string:
$s .= '<br /><hr />';
to
$s .= '<hr />';
I.e. you need to change value from <br /><hr />
to <hr />
- same value but without <br />
line break tag.