man: drop unused <authorgroup> tags from man sources

man: drop unused <authorgroup> tags from man sources

Docbook styles required those to be present, even though the templates that we
use did not show those names anywhere. But something changed semi-recently (I
would suspect docbook templates, but there was only a minor version bump in
recent years, and the changelog does not suggest anything related), and builds
now work without those entries. Let's drop this dead weight.

Tested with F26-F29, debian unstable.

$ perl -i -0pe 's/\s*<authorgroup>.*<.authorgroup>//gms' man/*xml