fff-batman-adv: Enable bridge loop avoidance (BLA)

Based on the documentation, BLA will only help if two nodes
are connected via WiFi AND via Ethernet cable on CLIENT ports!
For a "correct" setup, e.g. WiFi and BATMAN port connection,
BLA won't have an effect.

Since the former case is possible and there are no known
drawbacks, we enable BLA.

This removes the line to disable BLA, so it will be enabled as
B.A.T.M.A.N. enables it by default.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>