netlabel: Add support for removing a CALIPSO DOI.

      Remove a specified DOI through the NLBL_CALIPSO_C_REMOVE command.
It requires the attribute:
 NLBL_CALIPSO_A_DOI.

Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>