qapi/migration.json: Remove a variable that doesn't exist in example

Remove the "active" variable in example for query-colo-status.
It is a doc bug from commit f56c0065

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190303145021.2962-6-chen.zhang@intel.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>