lantiq: nand: drop ubifs images

lantiq: nand: drop ubifs images

Users are confused which image type they should use and there are more
drawbacks than adavantages in using a r/w ubifs rootfs in constrast to
a read-only squashfs rootfs like:

 - less available free flash space due to better compression of squashfs
   images
 - no support for factory reset due to r/w filesystem
 - possibility to break failsafe due to r/w filesystem

Therefore, drop support for r/w ubifs rootfs images.

Signed-off-by: Mathias Kresin <dev@kresin.me>