appdata: Remove documentation license
The code in this project is covered by GPL-2.0+. The documentation is covered by CC-BY-SA-3.0. As outlined on the GNOME Software wiki, combining these two licenses with 'and' produces incorrect results, with GNOME Software considering the project as a whole to be non-free (since OSI does not consider CC-BY-SA-3.0 to be a free license for software).
Instead, list only the code's license in the appdata. This is recommended by the appstream specification, and causes Software to correctly label this application as free software.
Fixes: #43 (closed)
https://gitlab.gnome.org/GNOME/gnome-software/-/wikis/software-metadata#user-content-how-to-fix-incorrect-licensing-information https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-project_license