batman-adv: Drop "experimental" from BATMAN_V Kconfig

The Kconfig option BATMAN_ADV_BATMAN_V is now enabled by default when the
BATMAN_ADV is enabled. A feature which is enabled by default for a module
should not be considered experimental.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>