Clean-up \Drupal\system\Plugin\ImageToolkit\GDToolkit when core require php 8.0

Problem/Motivation

Since PHP 8.0 GD no longer using resources, in parent issue there was introduced TODO to remove that method #3156887-10: \Drupal\system\Plugin\ImageToolkit\GDToolkit needs to support \GdImage objects for PHP 8 compatibility

Also checks with is_resource() should be removed

Proposed resolution

Remove the __destruct() method as imagedestroy() is no-op for 8.0+
Clean-up other methods which has shim to image when it was resource

Remaining tasks

create a patch, commit

User interface changes

no

API changes

TBD

Data model changes

no

Release notes snippet

no