Help: Avoid extra 1-pixel frame in Help widgets

For QTextBrowser and QLiteHtmlWidget based viewers

Change-Id: I0e8003b555b15e8eca239c59e657d9cf417f2d58
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/textbrowserhelpviewer.cpp

@@ -267,6 +267,7 @@ TextBrowserHelpWidget::TextBrowserHelpWidget(TextBrowserHelpViewer *parent)
     : QTextBrowser(parent)
     , m_parent(parent)
 {
+    setFrameShape(QFrame::NoFrame);
     installEventFilter(this);
     document()->setDocumentMargin(8);
 }