cleanup: remove redundant HTTPClient definition (#643)

This counts as a follow-up cleanup as part of doing
https://github.com/ooni/probe/issues/1885.

internal/measurex/http.go

@@ -193,25 +193,17 @@ type httpTransportBody struct {
 	io.Closer
 }
 
-// HTTPClient is the HTTP client type we use. This interface is
-// compatible with http.Client. What changes in this kind of clients
-// is that we'll insert redirection events into the WritableDB.
-type HTTPClient interface {
-	Do(req *http.Request) (*http.Response, error)
-	CloseIdleConnections()
-}
-
 // NewHTTPClient creates a new HTTPClient instance that
 // does not automatically perform redirects.
 func NewHTTPClientWithoutRedirects(
-	db WritableDB, jar http.CookieJar, txp model.HTTPTransport) HTTPClient {
+	db WritableDB, jar http.CookieJar, txp model.HTTPTransport) model.HTTPClient {
 	return newHTTPClient(db, jar, txp, http.ErrUseLastResponse)
 }
 
 // NewHTTPClientWithRedirects creates a new HTTPClient
 // instance that automatically perform redirects.
 func NewHTTPClientWithRedirects(
-	db WritableDB, jar http.CookieJar, txp model.HTTPTransport) HTTPClient {
+	db WritableDB, jar http.CookieJar, txp model.HTTPTransport) model.HTTPClient {
 	return newHTTPClient(db, jar, txp, nil)
 }
 
@@ -241,7 +233,7 @@ type HTTPRedirectEvent struct {
 var ErrHTTPTooManyRedirects = errors.New("stopped after 10 redirects")
 
 func newHTTPClient(db WritableDB, cookiejar http.CookieJar,
-	txp model.HTTPTransport, defaultErr error) HTTPClient {
+	txp model.HTTPTransport, defaultErr error) model.HTTPClient {
 	return netxlite.WrapHTTPClient(&http.Client{
 		Transport: txp,
 		Jar:       cookiejar,