`category_description()` should be taxonomy-agnostic.

This change reinstates the previous de facto behavior of `category_description()`.
See [40979], [42364]. Because `term_description()` no longer passes `$taxonomy` to
`get_term_field()`, the parameter is no longer needed and has been deprecated.

Fixes #42605. See #42771.
Built from https://develop.svn.wordpress.org/trunk@42368


git-svn-id: https://core.svn.wordpress.org/trunk@42197 1a063a9b-81f0-0310-95a4-ce76da25c4cd