cmd: Kconfig: Fix CMD_UBIFS dependencies

Remove the ARCH_SUNXI and RBTREE dependencies.
CMD_UBIFS already gets RBTREE from CMD_UBI (from MTD_UBI), and should
the first become independant from the latter, there would likely be a
dependency on MTD_UBI anyway.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Jagan Teki <jagan@openedev.com>
Signed-off-by: Karl Beldan <karl.beldan-ext@sagemcom.com>
Reviewed-by: Tom Rini <trini@konsulko.com>