Remove some old and hard to maintain effects

Relevant kwin mailing list discussion: https://mail.kde.org/pipermail/kwin/2021-May/005222.html

With the ongoing scene redesign, these effect need to be rewritten. However, given that they are not used widely based on support information from various bug reports, their huge code size and our available man power being sparse, the most reasonable thing is to drop them (for now), unfortunately.

With !1062 (merged), it should be possible to re-write these effects using QML (+ maybe EffectQuickView). It would be cool to see effects such as Desktop Cube re-implemented using QtQuick 3D. However, kdeplasma-addons would be a better place for them as the maintenance burden will be distributed more evenly that way.

cc #30