This currently causes issues with e.g. appstream-util validation:
$ appstream-util validate-relax org.sigrok.PulseView.appdata.xml
org.sigrok.PulseView.appdata.xml: FAILED:
• tag-invalid : <project_group> is not valid
Validation of files failed
This is considered an appstream/tool bug, but for now we drop the tag in
order it to avoid issues.
https://github.com/hughsie/appstream-glib/issues/140