Revert "gluon-mesh-batman-adv: reenable batman-adv multicast optimizations"

This reverts commit 9b1eb40f.

With the batman-adv v2019.2 upgrade reverted (c1a77339), the batman-adv
multicast-to-multi-unicast feature is not available yet. Without that it is
going to be very unlikely of the batman-adv multicast optimizations to
take effect. E.g. some outdated nodes would disable it.

To avoid confusion and diversion with a few communities having it enabled
and most implicitly deactivated, just deactivate it for all for now
until batman-adv is updated to v2019.2 or greater again.

package/gluon-mesh-batman-adv/luasrc/lib/gluon/upgrade/310-gluon-mesh-batman-adv-mesh

@@ -28,7 +28,7 @@ else
 		gw_sel_class = gw_sel_class,
 		hop_penalty = 15,
 		routing_algo = routing_algo,
-		multicast_mode = true,
+		multicast_mode = false,
 	})
 end