get_all_category_ids() only used by a deprecated function

[Latz]

The function "get_all_category_ids()" (category.php) is only used by the deprecated function "get_category_children". I got two ideas for the future of the function:

a) also deprecate it

b) create a function "get_all_taxonomy_ids($taxonomy)". Since it seems to be a common problem for programmers to get e.g. all tag ids I would prefer this.

The actual querying of the ids is done via get_terms() anyway:

$cat_ids = get_terms( 'category', array('fields' => 'ids', 'get' => 'all') );

The only benefit of get_all_category_ids() is that it caches the ids. However, that cache doesn't seem to be invalidated anywhere, so it's buggy.

Therefore, I say we deprecate it.

Hi guys-- here's a patch file, would appreciate if someone could skim it quickly to make sure I did it correctly.

the function is simplified here: #21760, although not completely deprecated