gnttab: drop GNTMAP_can_fail

There's neither documentation of what this flag is supposed to mean, nor
any implementation. Commit 4d45702cf0 ("paging: Updates to public
grant table header file") suggests there might have been plans to use it
for interaction with mem-paging, but no such functionality has ever
materialized. With this, don't even bother enclosing the #define-s in a
__XEN_INTERFACE_VERSION__ conditional, but drop them altogether.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/include/public/grant_table.h

@@ -628,9 +628,6 @@ DEFINE_XEN_GUEST_HANDLE(gnttab_cache_flush_t);
 #define _GNTMAP_contains_pte    (4)
 #define GNTMAP_contains_pte     (1<<_GNTMAP_contains_pte)
 
-#define _GNTMAP_can_fail        (5)
-#define GNTMAP_can_fail         (1<<_GNTMAP_can_fail)
-
 /*
  * Bits to be placed in guest kernel available PTE bits (architecture
  * dependent; only supported when XENFEAT_gnttab_map_avail_bits is set).