[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