Drop EffectsHandler::effectsConfig

Summary:
The effectsConfig is no longer used by any Effect. ScriptedEffects use
KWin's KSharedConfigPtr and the builtin effects all have a kcfg with
kwinrc being hard coded.

The effectsConfig had the big disadvantage that it hardcoded kwinrc
while the rest of kwin uses a KSharedConfigPtr which might not point to
kwinrc.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3384