And stop explicitly installing it as a Cabal data file. It was
previously used by the about dialogue, but since the license is now
boring LGPL 2.1 (or later) we use GtkAboutDialog's built-in support for
that license instead.

Since we no longer depend on any GPL library, we can proclaim just LGPL.
Specifying a valid SPDX license string will also make it easier
for distros to automatically check if the project is redistributable.

These are no longer used. This simplifies the license situation, too.

I'm okay with the binaries being GPLv3. Let's just say that they are.