Licenses - why PublicDomain?

ref: !682 (merged)

# an F-Droid addition, until we can enforce a better option
APPROVED_LICENSES.append("PublicDomain")

But reading:

• https://opensource.org/node/878
• https://www.fsf.org/blogs/rms/public-domain-manifesto
• https://wiki.spdx.org/view/Legal_Team/Decisions/Dealing_with_Public_Domain_within_SPDX_Files

...does not conclude that PublicDomain is OSI or FSF approved