nvdimm/namespace: Delete nd_namespace_blk

Now that none of the configuration paths consider BLK namespaces, delete
the BLK namespace data and supporting code.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/164688417727.2879318.11691110761800109662.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>