gnttab: drop unused GNTCOPY_can_fail

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/public/grant_table.h
+++ b/xen/include/public/grant_table.h
@@ -449,8 +449,6 @@ DEFINE_XEN_GUEST_HANDLE(gnttab_transfer_
 #define GNTCOPY_source_gref       (1<<_GNTCOPY_source_gref)
 #define _GNTCOPY_dest_gref        (1)
 #define GNTCOPY_dest_gref         (1<<_GNTCOPY_dest_gref)
-#define _GNTCOPY_can_fail         (2)
-#define GNTCOPY_can_fail          (1<<_GNTCOPY_can_fail)
 
 struct gnttab_copy {
     /* IN parameters. */