tty: serial: netx: Delete driver

The Netx ARM machine was deleted from the kernel. This driver
had no users and has to go.

Cc: Robert Schwebel <r.schwebel@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190722065146.4844-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>