Remove all links in post content

Not sure about the database but you can do it easily with the_content filter, just add the code below to your current (child) theme functions.php file:

add_filter( 'the_content', 'misha_remove_all_a' );
function misha_remove_all_a( $content ){

    return preg_replace('#<a.*?>(.*?)</a>#is', '\1', $content);

}