cxl/pmem: Delete unused nvdimm attribute

cxl/pmem: Delete unused nvdimm attribute

While there is a need to go from a LIBNVDIMM 'struct nvdimm' to a CXL
'struct cxl_nvdimm', there is no use case to go the other direction.
Likely this is a leftover from an early version of the referenced commit
before it implemented devm for releasing the created nvdimm.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220624041950.559155-19-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>