remove all those buttons and leave only the box to write

Hi,
you can replace the TinyMCE editor with a simple Textarea field by adding the code below in your theme functions.php file

add_filter( "adverts_form_load", function( $form ) {
  if( $form['name'] != "advert" ) {
    return $form;
  }
  foreach( $form["field"] as $key => $field ) {
    if( $field["name"] == "post_content" ) {
        $form["field"][$key]["mode"] = "plain-text";
    }
  }
  return $form;
});

thanks