atmel-qt5-demo-image: remove the postprocess function as it is for qt4e

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
atmel-qt4e-demo-image: make postprocess command work again

In the daisy build, the rootfs files are in {IMAGE_ROOTFS}. So we need to
go that folder before we do any clean up.

Otherwise postprocess() will do the clean up under folder:
  tmp/work/sama5d3xek-poky-linux-gnueabi/atmel-qt4e-demo-image/1.0-r3/atmel-qt4e-demo-image-1.0

As there is nothing in that folder, so clean up function doesn't works

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>