batman-adv: remove network coding support

batman-adv: remove network coding support

The Network Coding feature, introduced in 2013, is based on the master
thesis "Inter-Flow Network Coding for Wireless Mesh Networks". It relies on
the assumption that neighboring mesh nodes can reliably overhear each
other's transmissions in promiscuous mode, allowing packets to be combined
to reduce forwarding overhead.

This assumption no longer holds for modern wireless mesh networks, which
are heterogeneous and make overhearing increasingly unreliable. Factors
such as multiple spatial streams, varying data rates, beamforming, and
OFDMA all prevent nodes from consistently overhearing each other. The current
implementation in batman-adv is not able to detect these conditions and would
require a more complex layer beyond its neighbor discovery process to do so.

In addition, the feature has been unmaintained for years and is discouraged
for use. None of the current maintainers have the required test
setup to verify its functionality, and known issues remain in its data
structures (reference counting, RCU usage, and cleanup handling). Its
continued presence also blocks necessary refactoring of the core originator
infrastructure.

Remove this obsolete and unmaintained feature.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Martin Hundebøll <martin@hundeboll.net>
Acked-by: Marek Lindner <marek.lindner@mailbox.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>