Remove "What's This" window decoration button by default
The "What's This?" feature has been transformed into an expandable tooltip in frameworks/kxmlgui!45 (merged).
Now the text is shown by pressing the Shift key while one of these expandable tooltips is visible. As a result, there is no longer a need for a second redundant method of invoking the feature, so let's remove the button to do so from the default set of titlebar buttons.
We can probably remove this functionality entirely in Plasma 6, but for now simply removing it from the titlebar by default seems sufficient, to preserve the ability of people who still like it to continue using it.