The TabControl isn't meant to have an 'x' that closes the tab. macOS just simply doesn't have a control to do this. macOS Sierra however did come out with tabs on an application level that you can take advantage of instead, but it can't be used as a control within your app.
Good news is, there's a new DocumentControl
that is coming for Eto 2.4 release, which should do what you are looking for.
Hope this helps!