Staging: batman-adv: Remove CHANGELOG

The changelog is only generated on standalone releases. Thus it has no
real value for the in-kernel version of batman-adv.

Reported-by: Abraham Arce <abraham.arce.moreno@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>