When inserting an attachment into a WordPress post, we can customize how it should be displayed. There are three settings; Alignment, Link To and Size. Here’s how we can tell WordPress to select None as the default value of Link To:

/**
 * Set default media link to 'none'
 *
 * @param   string $value Option value
 * @wp_hook filter pre_option_image_default_link_type
 * @return  string
 */
function _kucrut_image_default_link_type( $value ) {
    return 'none';
}
add_filter( 'pre_option_image_default_link_type', '_kucrut_image_default_link_type' );

As of WordPress 4.0, there are four values available: file (default), post, custom and none.