Remove hack to enforce message window visibility on X11

Don't enforce bypassing window manager when we are about to show message window - if we detect that there is a full screen window currently being displayed. A comment in code says that this was necessary to have our window shown in such a case, at the cost of this window not having decoration. While the latter is true (additionally the message window is not managed by kwin at all, so it can't be moved, its KeepAbove property can't be disabled, etc. - so the side effects are rather unpleasant), the former seams not (not anymore at least - current kwin versions show our window under such circumstances).

Tested with current kwin built from master as well as 5.27 (~2 years old). The code in question was introduced in 2008 (77b74653), kwin's behaviour could be different back then.