normal/main: Unregister commands on module unload

normal/main: Unregister commands on module unload

When the normal module is loaded, the normal and normal_exit commands
are registered but aren't unregistered when the module is unloaded. We
need to add calls to grub_unregister_command() when unloading the module
for these commands.

Fixes: CVE-2025-61663
Fixes: CVE-2025-61664
Reported-by: Alec Brown <alec.r.brown@oracle.com>
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>