Move batch callback to the form class

Updated the UI form for removing taxonomy terms.
one batch item = one taxonomy term.

apart from that,
added #required attribute to the vocabulary field to fix https://www.drupal.org/project/taxonomy_delete/issues/3341213 💬 configuration form performing without selecting any option from the vocabulary field. RTBC issue.
added validation for taxonomy vocabularies to fix https://www.drupal.org/project/taxonomy_delete/issues/3341117 ✨ Improve UX when no taxonomy RTBC issue.

AstonVictor → closed merge request !6

The MR is merged.

Automatically closed - issue fixed for 2 weeks with no activity.