normal: Remove variables hooks on module unload

normal: Remove variables hooks on module unload

The normal module does not entirely cleanup after itself in
its GRUB_MOD_FINI() leaving a few variables hooks in place.
It is not possible to unload normal module now but fix the
issues for completeness.

On the occasion replace 0s with NULLs for "pager" variable
hooks unregister.

Fixes: CVE-2025-0622
Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name normal-Remove-variables-hooks-on-module-unload.patch