net: l2: ieee802154: simple address mode removed from spec

See IEEE 802.15.4-2020, 7.2.2.11 Source Addressing Mode field and
7.2.2.9 Destination Addressing Mode field, table 7-3: The
previously deprecated "Simple addressing mode" was removed
from the spec.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>