Fix to not send Proxy-Connection

Proxy-Connection is a non-standard header people started using because they misunderstood HTTP. It never makes sense for a Proxy to _send_ that header to either a parent proxy or an origin, under any circumstances.

We should remove the logic in ATS that does, and either send `Connection:` or nothing, as appropriate.

(Though it probably makes sense to use it, if we receive it.)

See:
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
https://github.com/apache/trafficserver/blob/926dd71/proxy/http/HttpTransact.cc#L6763