tools/pygrub: Remove unnecessary hypercall

tools/pygrub: Remove unnecessary hypercall

There's a hypercall being issued in order to determine whether PV64 is
supported, but since Xen 4.3 that's strictly true so it's not required.

Plus, this way we can avoid mapping the privcmd interface altogether in the
depriv pygrub.

This is part of XSA-443 / CVE-2023-34325

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit f4b504c6170c446e61055cbd388ae4e832a9deca)