nbd: Remove x-nbd-server-add-bitmap

Now that nbd-server-add can do the same functionality (well, other
than making the exported bitmap name different than the underlying
bitamp - but we argued that was not essential, since it is just as
easy to create a new non-persistent bitmap with the desired name),
we no longer need the experimental separate command.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190111194720.15671-7-eblake@redhat.com>