observer: remove TCP prober

[jsha]

It's unused per #8621.

[github-actions]

@jsha, this PR appears to contain configuration and/or SQL schema changes. Please ensure that a corresponding deployment ticket has been filed with the new values.

[aarongable]

This removes the only use of obsclient.Dialer() outside of the TLS prober. I think we can simplify the obsclient package as part of this.

[jsha]

Good suggestion. Tried this, but noticed that obsclient.Client() also calls Dialer(). So if I moved the implementation of Dialer() into the probers/tls package, I'd also have to duplicate it internally to Client(). It's a trivial amount of code so the duplication is not bad from that perspective, but it's nice to unify the comment in a single place, so I'm inclined to keep obsclient.Dialer().

[beautifulentropy]

Wow, I expected there to be an entry in cmd/boulder-observer/README.md for this, but nope!