mac802154: move interface del handling in iface

This patch moves and rename the mac802154_del_iface function into
iface.c and rename the function to ieee802154_if_remove which is a similar
naming convention like mac80211.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>