kirkwood: remove obsolete ubinize.cfg file

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/kirkwood/image/ubinize.cfg

-[rootfs]
-# Volume mode (other option is static)
-mode=ubi
-# Source image
-image=root.ubifs
-# Volume ID in UBI image
-vol_id=0
-# Allow for dynamic resize
-vol_type=dynamic
-# Volume name
-vol_name=rootfs
-# Autoresize volume at first mount
-vol_flags=autoresize