Problem/Motivation
1. As stated in #2751001: Expose configuration for Relation, users, such as site builders, are unable to control the visibility of Relations for a term via the Manage form display page or the taxonomy edit page. Some taxonomies may not require parent-child relations and therefore would not require the Relations for a term to be displayed on the create/edit page.
2. Related to the first point, being unable to control the visibility of Relations for a term, there is an issue when having a taxonomy with large sets of terms. Due to the backend form code of ensuring that A term can't be the child of itself, nor of its children, \Drupal\taxonomy\TermStorageInterface::loadTree() and \Drupal\taxonomy\TermStorageInterface::loadParents() are called, which can cause memory errors, WSOD, etc. Based on where this process is, a hook_form_alter() is too late to prevent these functions from being called. There is an if statement and a comment that states that switching taxonomy.settings:override_selector to TRUE would skip the calling of these functions, but this isn't very user friendly to a site builder and was not super easy to decipher prior to digging in the code.
Proposed resolution
1. Convert the Parent and Weight Base Fields to be available and controlled via the Manage form display page for Taxonomy Terms in core/modules/taxonomy/src/Entity/Term.php
2. Move the Relations form functionality to a hook_form_alter() in taxonomy.module
3. Remove the Relations form functionality from core/modules/taxonomy/src/TermForm.php
Remaining tasks
Run tests and potentially modify test cases