remove_category.php

function remove_category( $string, $type ) {
    if ( $type != 'single' && $type == 'category' && ( strpos( $string, 'category' ) !== false ) )  {
        $url_without_category = str_replace( "/category/", "/", $string );
        return trailingslashit( $url_without_category );
    }
    return $string;
}

add_filter( 'user_trailingslashit', 'remove_category', 100, 2);