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