Taxonomy: Make some adjustments to handling default terms for custom taxonomies:

* Move default term assignment from `wp_set_object_terms()` to `wp_insert_post()`.
* Make sure the passed taxonomy list overwrites the existing list if not empty.
* Remove the default term option on `unregister_taxonomy()`.
* Prevent deletion of the default term in `wp_delete_term()`.

Props enrico.sorcinelli, TimothyBlynJacobs.
See #43517.
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