Remove close button from default toolbar
It's redundant: - If the current document is the only one, you can use the window's close button to close the window. - If the current document is one of many, you can use the close button on that document's tab to close it. The only non-redundant workflow that this toolbar button supports is the workflow of closing the last open document to get back to the welcome screen again, which seems like quite a weird niche thing to do. Accordingly, we can reduce some visual clutter on the toolbar by removing this button and the separators around it.