Remove old show_last_update argument and related code from wp_dropdown_categories

[solarissmoke]

In wp_dropdown_categories() and the related walker class there is reference to a show_last_update argument which is linked to a last_update_timestamp property for category objects. That property was dropped about 5 years ago ([5525])!

Patch removes these. I haven't bothered adding _deprecated_argument() calls, seeing as setting the argument doesn't actually work (you just get PHP warnings), and hasn't for a really long time.

[nacin]

Works for me.

[duck_]

In [19927]:

Remove old show_last_update/include_last_update_time/show_date code from category template functions. Props solarissmoke. Fixes #19953.

[duck_]

In [19928]:

Remove use of nonexistent include_last_update_time arg from get_terms() call. See #19953.

[chriscct7]

#16872 was marked as a duplicate.