kirkwood: drop unnecessary argument from *_UBIFS_OPTS variables

The '-U' aka '--squash-uids' is already used in Image/mkfs/ubifs function.

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38898 3c298f89-4303-0410-b956-a3cf2f4a3e73