UBIFS: remove dead code

UBIFS assumes that @c->min_io_size is 8 in case of NOR flash. This
is because UBIFS alignes all nodes to 8-byte boundary, and maintaining
@c->min_io_size introduced unnecessary complications.

This patch removes senseless constructs like:

if (c->min_io_size == 1)
	NOR-specific code

Also, few commentaries amendments.

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