The dax_pmem driver was implementing an empty ->remove() method to satisfy the nvdimm bus driver that unconditionally calls ->remove(). Teach the core bus driver to check if ->remove() is NULL to remove that requirement. Reported-by: Johannes Thumshirn <jthumshirn@suse.de> Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de> Signed-off-by: Dan Williams <dan.j.williams@intel.com>