Log Message:
gr-osmosdr: drop some unused dependencies (osmo-sdr and libmirisdr)

Upstream doesn't build support for these devices by default anymore:
> The actual osmosdr had only a very limited production run many years
> ago and is therefore of little interest to most gr-osmosdr users.
> Mirisdr was experimental at best, because the dongles were never
> widely avaialble outside of Japan and had multiple unconnected antenna
> ports.