batman-adv: remove useless assignment in tt_local_add()

The flag field of the tt_local_entry->common structure in
tt_local_add() is first assigned NO_FLAGS and then TT_CLIENT_NEW so
nullifying the first operation. For this reason it is safe to remove
the first assignment.

This was introuduced by ("batman-adv: keep local table consistency for
further TT_RESPONSE")

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>