nvdimm/namespace: Delete blk namespace consideration in shared paths

Given is_namespace_blk() is never true outside of the NVDIMM unit tests
delete the support from namespace device management.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/164688417214.2879318.4698377272678028573.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>