x11-toolkits/kf5-kdesignerplugin: remove obsolete dependencies
Back in 2019...
... the webkit designer plugin was removed in [1] ... the other plugins were dropped in [2]
[1] https://invent.kde.org/frameworks/kdesignerplugin/-/commit/729f723f76751f106e803903cfd9b16d6fa350f0
[2] https://invent.kde.org/frameworks/kdesignerplugin/-/commit/cc27fa4fa59badca27d55baf17f73dcef2b8ddfe