net: remove netx ethernet driver

The ARM netx platform got removed in 5.3, so this driver
is now useless.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>