Don't create QWhatsThis when user presses showContextHelp button

Summary:
I don't know what the idea of entering QWhatsThis mode was, but the code
is pretty much dead. There is no QWhatsThis window shown as KWin does
not have any UI or help. The window which eventually gets shown in
context help is provided by the client application.

Furthermore KWin never leaves the QWhatsThis mode. At least one code
path entering in leave is dead in general and one at least on Wayland
(but probably also on X11 as the window never gets shown).

The show context help functionality works fine without the QWhatsThis
handling, so let's remove it.

I tried to blame what was the idea behind it, but it was unchanged since
decades and blame ended in a code moving commit.

Test Plan: Used show context help (kcmshell5 --platform xcb kwinoptions)

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7398