proxinstall, common: remove deprecated fletcher2 as zfs checksum algorithm

proxinstall, common: remove deprecated fletcher2 as zfs checksum algorithm

Fletcher-2 has long been deprecated and should not be used anymore
[0][1], so we probably should not offer it anymore too. It's been
deprecated since at least over 3 years, beyond that it's hard to find
an exact date.

[0]: https://openzfs.github.io/openzfs-docs/Basic%20Concepts/Checksums.html#checksum-algorithms
[1]: https://people.freebsd.org/~asomers/fletcher.pdf

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