ARM: delete netx machine

After discussing with the subarch maintainers and Hilscher,
we concluded that the netx subarchitecture (Netx 100/500)
is no longer maintained or tested, and noone will miss it
if we delete it. So delete it.

There is a newer Netx 4000 architecture which we may see
included at some point, but this will be supported using
the standard multiplatform and devicetree mechanisms and is
easier to develop from scratch.

Cc: Michael Trensch <MTrensch@hilscher.com>
Acked-By: Robert Schwebel <r.schwebel@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>