batman-adv: drop the vestigial conffiles entry

batman-adv: drop the vestigial conffiles entry

/etc/config/batman-adv is registered as a conffile although the
package never installs it: files/ only ships the uci-defaults
migration script and the netifd proto handlers. The migration script
itself removes /etc/config/batman-adv as its last step once the
settings have been moved into /etc/config/network, so the entry is
left over from the layout that predates batadv_hardif.

Reported-by: openwrt-ai[bot]
Signed-off-by: Josef Schlehofer <redacted>
Co-authored-by: Claude Fable 5 <redacted>