It turns out this navigation bar only appears if it detects that the request is coming from an Android phone. So we can simply change the user agent to something else and this navigation bar will disappear.
WebSettings settings = webView.getSettings();
settings.setJavaScriptEnabled(true); //Required for Google Groups
settings.setUserAgentString("MyApplication Android");
webView.loadUrl("https://groups.google.com/forum/m/");