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