[kcmkwin/compositing] Delete leftover after removed effects

Summary:
We don't have anymore the following effects:

* Box Switch (was removed in 4.11);
* Explosion (was removed in 4.11);
* Outline (was removed in 4.11);
* Scale In (will be removed in 5.14);
* Taskbar Thumbnails (was removed in 4.95).

So, keywords for those effects should not be among search suggestions in
System Settings.

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14765