sama5d3_xplained.conf: remove IMAGE_POSTPROCESS_COMMAND

The method is only defined in the demo images, so trying to build any
other image will result in:

/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/work/sama5d3_xplained-angstrom-linux-gnueabi/systemd-image/1.0-r0/temp/run.do_rootfs.14199:
line 436: sama5d3_xplained_rootfs_postprocess: command not found

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>