ARM: ixp4xx: Delete the D-Link DSM-G600 boardfiles

This board is replaced with the corresponding device tree.

Cc: Michael-Luke Jones <mlj28@cam.ac.uk>
Cc: Rod Whitby <rod@whitby.id.au>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Michael Westerhof <mwester@dls.net>
Cc: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>