batman-adv: Remove ethtool .get_settings stub

The .get_settings function pointer and the related API was deprecated.
Fortunately, batman-adv is a virtual interface and never provided any
useful information via .get_settings. The stub can therefore be
removed.

This also avoids that incorrect information is shown in ethtool about the
batadv interface.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>