Revert "libnvdimm, of_pmem: workaround OF_NUMA=n build error"

With commit df3f1264 ("libnvdimm, of_pmem: use dev_to_node() instead
of of_node_to_nid()") it is now possible to allow of_pmem to be built as
a module as originally implemented.

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