Remove `DID.Oldstyle`

[matheus23]

@expede and I have talked about the *.DID.Oldstyle.Types module recently. We were agreeing that we should just delete that code

I now saw that we're actually using it in production right now: We set the DNS TXT record for the server's DID to the Oldstyle Ed25519 DID. That's not really a problem for our clients, because they're just signing the very last UCAN off for the server in any format, they don't care. The server understands both formats. So I think we can "just" turn off the Oldstyle DID?

The only case I can think of where this would fail would be if our server would actually use it's DID to sign things, but I don't think that's the case.