Drop link-mso19/nexus-osciprime in preparation for release.

Drop link-mso19/nexus-osciprime in preparation for release.

These two drivers are currently unfinished and don't work, so disable
and "unhook" them for now in preparation of the next libsigrok release.

They're still in the git repository, but not hooked up to the build
system, so that they won't get detected or built, and also don't end up
in the release tarball.

Since link-mso19 is the only driver that currently requires libudev,
drop any reference to that, too.

It should be relatively easy to apply this patch in reverse after the
release to bring back both drivers.