gnttab: drop GNTMAP_can_fail

gnttab: drop GNTMAP_can_fail

There's neither documentation of what this flag is supposed to mean, nor
any implementation. Commit 4d45702cf0398 ("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>