Optimize `wp_delete_term()` for large object counts without a default term.

[dd32]

When deleting a term, the term has to be removed individually from each object it's connected to. When there exist a lot of objects, that can take some significant time.

The attached PR optimises wp_delete_term() slightly to simply call wp_remove_object_terms() when no default term is required, skipping the terms fetch/diff step which can significantly speed up the deletion process.

This was encountered on WordPress.org Support Forums while trying to remove a topic tag with 10,000+ threads, at that scale the fetch/diff process was much slower than simply removing the term.

When no default term exists for a taxonomy and we're deleting a term, skip fetch/diff/set, just remove the term directly from all objects.

Trac ticket: https://core.trac.wordpress.org/ticket/52549

[SergeyBiryukov]

In 50389:

Taxonomy: Optimize wp_delete_term() for large object counts without a default term.

When deleting a term, it has to be removed individually from each object it's connected to, which can take some significant time when there are a lot of objects.

By calling wp_remove_object_terms() when no default term is required, we can skip the terms fetch/diff step and significantly speed up the deletion process.

Props dd32.
Fixes #52549.

Merged with changeset https://core.trac.wordpress.org/changeset/50389