Use 'the_category_rss' template function in Atompub backend.

[znarfor]

Fundamentaly, wp-app.php and feed-atom.php outputs are handled separetely, resulting in inconsistencies. I understand this can be difficult to fix completely, but we can move forward with simple and easy steps.

The current problem is that tags are not displayed in wp-app.php output. Replacing the current code by the standard the_category_rss('atom') fix that, avoid to repeat code and should be more straightforward.

Minor style tweaks to original patch

(In [10727]) Use 'the_category_rss' template function in Atompub backend. Props znarfor, josephscott. fixes #8829