[fix] Fixing getArticleRoute() calls in finder content plugin

There is no need to use catslug in Joomla 3.x because:
1. Core content component doesn't use catslug in article URLs;
2. Process link function - **ContentHelperRoute::getArticleRoute** - requires only catid.
`/components/com_content/helpers/route.php`
`public static function getArticleRoute($id, $catid = 0, $language = 0)`

See more #4371 and #5276

After this PR all URLs to the articles must be identical as in content component.