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/ooapi/fake_test.go

@@ -41,6 +41,8 @@ func (c *FakeHTTPClient) Do(req *http.Request) (*http.Response, error) {
 	return c.Resp, nil
 }
 
+func (c *FakeHTTPClient) CloseIdleConnections() {}
+
 type FakeBody struct {
 	Data []byte
 	Err  error