batman-adv: Join batadv_purge_orig_ref and _batadv_purge_orig

The single line function batadv_purge_orig_ref has no function beside
providing the name used by other source files. This can also be done
simpler by just renaming _batadv_purge_orig to batadv_purge_orig_ref.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>