net: ieee802154: Remove old cc2520 AUTO_ACK assignments

CONFIG_IEEE802154_CC2520_AUTO_ACK started out as
CONFIG_TI_CC2520_AUTO_ACK, which was removed in commit 6b43821f
("net: Remove legacy Contiki based uIP stack"). The assignments were
later renamed in commit 573774a9 ("drivers/net/ieee802154: Change
configuration prefix").

Remove them.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>