In the roadmap of XDK, access to App Centre is going to be removed and according to the header on the App Centre page, all functionality that the page provides should be available under XDK.
If that is the case, how do I remove old projects from App Preview in XDK, I cannot see that option so I am having to use App Centre to remove them.
That option has not yet been added to App Preview or the XDK. That is why it is still present in App Center. When it becomes available through an alternate mechanism the App Center access will be removed.