[kcmkwin] Rename EffectsModel::m_effectsList to EffectsModel::m_effects
Summary: "List" suffix is redundant.
Test Plan: Compiles.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: davidedmundson, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18713