Currently I am working on a website which is using meta tag to render the page in IE9 mode [ BrowserMode: IE10, DocMode: IE9 Standards ]
I'm sorry to be the bearer of bad news, but this is not possible.
If you want to get rid of the close widget, your only option is to switch to standards mode.
In IE10 lots of CSS breaks in the page, so using
IE=Edgeis not a viable solution as of now ( may be in future this will be used once all the css is fixed, but I don't see this near future ).
So I guess it comes down to how badly you want to get rid of the X widget. If it's bugging you that badly then you will have to bite the bullet, fix your CSS, and switch to standards mode.
Otherwise, you'll just have to live it.
Sorry.