Delete unused functions from .module

Problem/Motivation

For Drupal 9/10+ the functions "user_mailman_register_form_user_register_form_alter," "user_mailman_register_form_validate," and "user_mailman_register_form_submit" were moved to src/Form/JoinListForm.php (to "buildForm," "validateForm," and "submitForm" respectively) and need to be removed from user_mailman_register.module.