net: adi: remove blackfin ethernet drivers

The blackfin architecture is getting removed, so the bfin_mac driver
is now obsolete.

Acked-by: Dominik Brodowski <linux@dominikbrodowski.net>
Acked-by: Aaron Wu <aaron.wu@analog.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>