katetabbar: do not set any icon when the document is modified

When splitting the view on a modified document the tab bar button in the new view would show a save icon, whereas the button in the old view wouldn't.

From what I saw the icon shouldn't appear since !628 (merged) (and that works fine except for this tiny edge case :-)). So I just removed the calls that set it.

Here's a screenshot of the tab bar before the change (above) and after it (below) with the Macintosh styling, though I also observed this on my work laptop with the Breeze style: