ubifs: Drop unnecessary setting of zbr->znode

in dbg_walk_index ubifs_load_znode is used to load the znode behind
a zbranch. ubifs_load_znode links the new child znode to the zbranch,
so doing it again is unnecessary.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>