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/httpx/jsonapi.go

@@ -10,8 +10,6 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/url"
-
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
 )
 
 // Logger is the definition of Logger used by this package.
@@ -40,10 +38,6 @@ type Client struct {
 	// Logger is the logger to use.
 	Logger Logger
 
-	// ProxyURL allows to force a proxy URL to fallback to a
-	// tunnel, e.g., Psiphon.
-	ProxyURL *url.URL
-
 	// UserAgent is the user agent to use.
 	UserAgent string
 }
@@ -93,11 +87,6 @@ func (c Client) NewRequest(ctx context.Context, method, resourcePath string,
 		request.Header.Set("Accept", c.Accept)
 	}
 	request.Header.Set("User-Agent", c.UserAgent)
-	// Implementation note: the following allows tunneling if c.ProxyURL
-	// is not nil. Because the proxy URL is set as part of each request
-	// generated using this function, every request that eventually needs
-	// to reconnect will always do so using the proxy.
-	ctx = dialer.WithProxyURL(ctx, c.ProxyURL)
 	return request.WithContext(ctx), nil
 }