relay: Stop DirPort self reachability test and stop publishing it

With #2667 (closed), we've now denied network re-entry for security reasons and thus relay self reachability test on the DirPort won't work anymore.

We should thus remove it at once and not stuck relays on this. Directory requests are happening on the ORPort since many years ago thus relay DirPort is less and less relevant. It is still important on directory authorities at the moment though but reachability test still won't work.

Talked to nickm about this and the highlights:

1. Client should ONLY use ORPort for directory content. We'll make an assessment on all supported versions of Tor to make sure.

2. DirCache should be fine without a DirPort according to the manpage but we should again assess that it is true on all supported versions.

 DirCache 0|1
     When this option is set, Tor caches all current directory documents
     except extra info documents, and accepts client requests for them.
     If DownloadExtraInfo is set, cached extra info documents are also
     cached. Setting DirPort is not required for DirCache, because
     clients connect via the ORPort by default. Setting either DirPort
     or BridgeRelay and setting DirCache to 0 is not supported.
     (Default: 1)

3. Make sure that directory and bridge authority still properly works without a DirPort selftest.

All this resulting in the removal of the DirPort selftest and entirely stop publishing it in the descriptor at once except for authorities.