ARM: mvebu: x530: Remove custom kwbimage.cfg

Commit ca1a4c86 ("mvebu: select boot device at SoC level") made it
unnecessary for the A385 boards to have their own kwbimage.cfg but as
the x530 was in flight at the time it was added with it's own
kwbimage.cfg. Remove the custom kwbimage.cfg as the SoC level file is
suitable.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>