tools: kwbimage: Don't parse PAYLOAD keyword

The PAYLOAD keyword does nothing. No code is using it and both mkimage
and kwbimage completely ignore it. It looks like a relict from the past.
The payload image itself can be specified only via -d parameter to
mkimage.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Chris Packham <judge.packham@gmail.com>