Remove custom logic for avoiding duplicate plugins

This is now done in KCoreAddons with frameworks/kcoreaddons@e99a8eaa
and consequently the logic is not needed anymore.

Task: https://phabricator.kde.org/T12154
1 job for !4 with work/plugin_cleanup in 2 minutes and 15 seconds (queued for 9 minutes and 34 seconds)
latest merge request