Use upstream's desktop file

[muelli]

We currently use our own .desktop file:

org.briarproject.Briar/org.briarproject.Briar.yaml

Line 45 in 42ca1f1

# The .desktop file could be extracted from the released deb package

But upstream generates .deb packages which contain a desktop file. We can extract the file from there.
Even better would be to generate the file from the sources, somehow.