net: Remove variables hooks when interface is unregisted

net: Remove variables hooks when interface is unregisted

The grub_net_network_level_interface_unregister(), previously
implemented in a header, did not remove the variables hooks that
were registered in grub_net_network_level_interface_register().
Fix this by implementing the same logic used to register the
variables and move the function into the grub-core/net/net.c.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Conflicts:
	grub-core/net/net.c

Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name net-Remove-variables-hooks-when-interface-is-unregisted.patch