cxl/memdev: Remove unused cxlmd field

This field was left over when the connection between the cxl_memdev and
cxl_mem was tighter.  It is no longer set nor used so remove it.[1]

Link: https://lore.kernel.org/r/CAPcyv4hcgh2gb8qsS_UXTBSGqYfMPnC6p5kkvNUjm+V6kVKM5g@mail.gmail.com/

Suggested-by:  <Jonathan.Cameron@huawei.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20211103234857.3689354-1-ira.weiny@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>