MAINTAINERS: Remove Sven Eckelmann from BATMAN ADVANCED

I cannot speak on behalf of the batman-adv developers due to conflicts
in the opinion about the ongoing development. The batman-adv module is
still maintained by Marek Lindner and Simon Wunderlich. Those are the
main persons behind the visions of batman-adv. Therefore, the state of
module hasn't changed.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Cc: b.a.t.m.a.n@lists.open-mesh.org
Signed-off-by: David S. Miller <davem@davemloft.net>