[KConfigLoader] Code cleanup

Summary:

• Drop redundant args to startElement()/endEelement()
• Use range-for
• Drop ConfigLoaderHandler methods that weren't used anywhere AFAICS and ConfigLoaderHandler is private API

Test Plan: make && ctest

Reviewers: Frameworks, apol, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27157