[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