Remove unused code for loading task switcher KCM

For all the task switchers on my system there is no KCM installed:

```bash
rg 'X-KDE-ParentComponents=(PlasmaXLight|org.kde.breeze.desktop|org.kde.breezedark.desktop|org.kde.breezetwilight.desktop|org.kde.breeze.desktop|org.kde.breezedark.desktop|org.kde.breezetwilight.desktop|org.kde.plasma.mycroft.bigscreen|small_icons|big_icons|compact|text|present_windows|thumbnail_grid|thumbnails|informative)'
```

As the docs state, this is only for buildin effects. Which means it is internal API.
Also I consider the choice of displaying a KCM *or* the preview odd, IMHO the preview
is the most important part.

I stumbled upon this when reviewing the metadata files as part of the preparation
for https://phabricator.kde.org/T14564.

Also this way of plugin loading is discouraged for performance reasons, because all the
plugins from the namespace have to be reopened to get their metadata.