There's neither documentation of what this flag is supposed to mean, nor
any implementation in the hypervisor.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -196,9 +196,6 @@ typedef union {
 #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).