Use std::find_if in EffectsHandlerImpl::isEffectLoaded
Summary:
Don't use raw loops. Use appropriate algorithm from STL to find out
whether effect with the given name is loaded.
(also, this change deletes duplicated logic)
Reviewers: KWin, broulik, romangg
Reviewed By: KWin, broulik, romangg
Subscribers: broulik, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D13836