UBIFS: remove custom list of superblocks

This is a clean-up of the power-cut emulation code - remove the custom list of
superblocks which we maintained to find the superblock by the UBI volume
descriptor. We do not need that crud any longer, because now we can get the
superblock as a function argument.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>