gluon-mesh-batman-adv: remove MTU workaround for VLAN-on-WLAN

We don't support VLANs on 11s interfaces, so the workaround can be dropped
with the IBSS support.

package/gluon-mesh-batman-adv/files/lib/gluon/core/mesh/setup.d/30-gluon-mesh-batman-adv

 #!/bin/sh
 
 if [ "$FIXED_MTU" -eq 0 ]; then
-	# In case on VLAN on IBSS, first set MTU of the underlying interface
-	for lower in /sys/class/net/"$IFNAME"/lower_*/wireless; do
-		lower="${lower%%\/wireless}"
-		lower="${lower##*\/lower_}"
-		ip link set dev "$lower" mtu 1536
-		break
-	done
-
 	ip link set dev "$IFNAME" mtu 1532
 fi