Mark let-if and ssh-agent-unix unavailable

[reynir]

let-if was a fun experiment, but I don't think it's seen any usage and I now think it shouldn't be used.

ssh-agent-unix was not meant to be published, but was published sort of by accident due to dune-release picking it up. Previous versions have been of poor quality, and I don't think it's very useful beyond small examples. I would also like to not have to maintain this package (still want to maintain ssh-agent though!)

See also the discussion in #23789 (comment)

[mseri]

I am letting this wait a moment to see if there are users.
There are surely no published packages using them

❯ opam list --depends-on=ssh-agent-unix --all
# Packages matching: depends-on(ssh-agent-unix) & any
# No matches found
❯ opam list --depends-on=let-if --all
# Packages matching: depends-on(let-if) & any
# No matches found

[mseri]

No complain here, I am going on with it