fs/ufs: remove unused ufs_super_block_third pointer

Pointer 'usb3' to struct ufs_super_block_third acquired via
ubh_get_usb_third() is never used in function
ufs_read_cylinder_structures().  Thus remove it.

Detected by Coverity: CID 139939.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Cc: Evgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>