batman-adv: Drop license boilerplate

All files got a SPDX-License-Identifier with commit 7db7d9f3
("batman-adv: Add SPDX license identifier above copyright header"). All the
required information about the license conditions can be found in
LICENSES/.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>