ipq806x: drop kernel/ rootfs partition images for the ZyXEL NBG6817

ipq806x: drop kernel/ rootfs partition images for the ZyXEL NBG6817

Since commit 8e9a59a6b924469c0061d4bd945ae0c6dbfdecfe (build: add
mkrasimage) a proper factory image can be provided for the ZyXEL
NBG6817, which can be flashed from the OEM firmware or used for
tftp based push-button recovery, therefore the previously used
partition images for mmcblk0p5-rootfs.bin and mmcblk0p4-kernel.bin
are no longer required (and using them properly has always been
slightly difficult and risky) and can now be dropped from the image
generation code.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>