Bluetooth: BAP: Shell: Delete uni group when all streams released

Previously we assumed only a single stream, and deleted
the unicast group on the first stream released.

Now we wait for all the streams to be released before
deleting the group.

This still assumes that all streams are initiated by us
as the client, and will not work if using the
BAP unicast client and server role concurrently.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>