Remove unused defines for NVME PostBuf.

These defines for the posting of buffers for nvmet target were not used.

Removing the unused defines.

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>