proxinstall, common: remove "off" as zfs checksum option

proxinstall, common: remove "off" as zfs checksum option

See also the thread at [0] for the initial discussion/idea.

Disabling checksums is considered an "extraordinarily bad idea" [1] (for
pretty obvious reason) and nobody should really ever use it.

Thus remove the option completely; just so that users cannot simply
disable checksum "for performance reasons" without knowing about the
implications of this.

As pointed out by Thomas, it can still be set to "off" after the
installation using the `zfs` tool, if really wanted.

[0] https://lists.proxmox.com/pipermail/pve-devel/2023-December/061188.html
[1] https://openzfs.github.io/openzfs-docs/Basic%20Concepts/Checksums.html#disabling-checksums

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>