remove category field

You can do that by adding the code below in your theme functions.php file it will do that

function customize_adverts_add( $form ) {
  if( $form['name'] != "advert" ) {
    return $form;
  }
  foreach( $form["field"] as $key => $field ) {
    if( $field["name"] == "advert_category" ) {
        unset( $form["field"][$key] );
    }
  }
  return $form;
}

Thanks.
Are you sure there’s nothing missing ?
You’re just declaring the function here.
Where is it called ?
I added this declaration in my functions.php file and nothing changed…

Finally I edited the function that adds my custom fields.
I just added the “unset” part:

function custom_adverts_form_load( $form ) {

    if( $form["name"] != "advert" ) {
        return $form;
    }

    // Here I add my custom fields
    // ...

    // removing category
    foreach( $form["field"] as $key => $field ) {
        if( $field["name"] == "advert_category" ) {
            unset( $form["field"][$key] );
        }
    }

    return $form;
}
add_filter( "adverts_form_load", "custom_adverts_form_load" );

And it’s working fine.

Another quick question:
can I switch the order of the fields ?
I’d like to have the image gallery after the ad description.
Is it possible ?

    ...
    foreach( $form["field"] as $key => $field ) {
        if( $field["name"] == "gallery" ) {
            $form["field"][$key]["order"] = 10;
        }
    }
    ...