batman-adv: Don't manually reattach hard-interface

The batadv_hardif_enable_interface is now only called from the callback
ndo_add_slave. This callback is only used by do_set_master in the rtnetlink
code which only does two things:

1. remove the net_device from its old master
2. add the net_device to its new batadv master

The code to replicate the first step in batman-adv is therefore unused
since the sysfs code was dropped.

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