Clear the cache when converting a category to a tag, so category is removed from the list of cats

[warwickp]

Props: Andy Skelton

When a category is converted to a tag, the category should be removed from the list of categories that populates the category list on /wp-admin/post-new.php

The converted category however persists in the cache, unless the cache is deleted, which this patch achieves.

[ryan]

I think that can use clean_term_cache() instead of clearing category_children and last_changed by hand. I'll look.

[ryan]

(In [10608]) Make sure cache is clean when converting cats to tags. fixes #9186