Use blank am_pm and t_fmt_ampm for de_AT and de_DE locales as apparently
they use 24-hour clock notation. The visible change is that w(1) now
uses 24-hour clock format as it checks for t_fmt_ampm presence.

PR: 231771
Submitted by: Christoph Schönweiler <public2016@hauptsignal.at>
Reviewed by: bapt
Approved by: kib (mentor, implicit)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D17945