cleanup(netx): remove unused proxy-via-context codepath (#367)

We always set the proxy explicitly now. So, let us remove this
extra bit of code we're not using.

Part of https://github.com/ooni/probe/issues/1507.

internal/engine/probeservices/probeservices.go

@@ -95,7 +95,6 @@ func NewClient(sess Session, endpoint model.Service) (*Client, error) {
 			BaseURL:    endpoint.Address,
 			HTTPClient: sess.DefaultHTTPClient(),
 			Logger:     sess.Logger(),
-			ProxyURL:   sess.ProxyURL(),
 			UserAgent:  sess.UserAgent(),
 		},
 		LoginCalls:    &atomicx.Int64{},