virtchnl: Remove unused VIRTCHNL_VF_OFFLOAD_RSVD define

Remove unused define that is currently marked as reserved. This will
open up space for a new feature if/when it's introduced. Also, there is
no reason to keep unused defines around.

Suggested-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>