Remove textproc/gnome-doc-utils.
It is abandonded upstream for years and is Python-2.7-only.
Adjust consumers, some only need a rebuild, others have their
documentation formerly created by gnome-doc-utils / xml2po removed.
Remove gnomedocutils from USE_GNOME
Submitted by: rene
Approved by: portmgr (mat), gnome (kwm)
Differential Revision: https://reviews.freebsd.org/D23381