Use KConfig::readEntry()'s default argument properly

Summary: Follow-on patch to D7440 to use KConfig::readEntry()'s default argument properly to reduce some unnecessary code

Test Plan: Tested in KDE Neon. no regressions; by default, all plugins are enabled if the key is not set, and if it is set, user changes to the list are remembered

Reviewers: Dolphin, elvisangelaccio, dfaure, apol

Reviewed By: Dolphin, elvisangelaccio, apol

Differential Revision: https://phabricator.kde.org/D8846