batman-adv: remove obsolete skb_reset_mac_header() in batadv_bla_tx()

Our .ndo_start_xmit handler (batadv_interface_tx()) can rely on having
the skb mac header pointer set correctly since the following commit
present in kernels >= 3.9:

"net: reset mac header in dev_start_xmit()" (6d1ccff6)

Therefore this commit removes the according, now redundant,
skb_reset_mac_header() call in batadv_bla_tx().

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>