UBIFS: Remove bogus assert

This assertion was only correct before UBIFS had xattr support.
Now with xattr support also a directory node can carry data
and can act as host node.

Suggested-by: Artem Bityutskiy <dedekind1@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>