Mixed Content with http://ajax.aspnetcdn.com/ajax/4.5.1/1/WebForms.js

I have a web application using ASP.NET 4. It displayed error below : Mixed Content: The page at 'https://... was loaded over HTTPS, but requested an insecure script 'http://ajax.aspnetcdn.com/ajax/4.5.1/1/WebForms.js'. This request has been blocked; the content must be served over HTTPS.

The mixed content probably due to below :

http://ajax.aspnetcdn.com/ajax/4.5.1/1/WebForms.js

http://ajax.aspnetcdn.com/ajax/4.5.1/1/MicrosoftAjax.debug.js

http://ajax.aspnetcdn.com/ajax/4.5.1/1/MicrosoftAjaxWebForms.debug.js

But I didn't used above in the project. And I have enable CDN=true at the Script Manager as below :

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true" EnableCdn="true"/> 

How should i troubleshoot it or solve it ? Any clues ?

Thanks.