Drop the ObscurringWindow functionality on desktop switching

Summary:
The ObscurringWindows are a hack to make it more unlikely that the
desktop wallpaper becomes visible while switching desktops. This was
introduced in 2001 with SVN revision 90111
(git d194f74314285932eb72e1dfecd2816b63647e4e).

In 2001 desktop switching was probably a costly action where it was not
possible to just reorder the desktop in one frame. Nowadays we use
mostly compositing and with compositing the ObscurringWindows are
already disabled. But even in the case that no compositing is used I
consider it as very unlikely that the desktop becomes visible during the
switching. Given that I think it's time to remove this legacy solution.

Test Plan: Compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7965