The cache bucket sits between the storage functions and the backend
storage. We only read from the storage once, so there is no need to cache
anything. The real purpose of the cache bucket is to keep the -EUCLEAN
information when a NAND block needs to be rewritten and to keep the read
buffers as long as the backend iterates over all buckets trying to find
the one we want to use.

This can be coded easier and more obvious in the backend code, so drop
the cache bucket.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>