Various minor cleanups in Kross

Summary:

• Remove language-specific tests
• loadLibrary: Make warning message more human-readable
• Move CMake code generating .h files into respective directories
• Do not unnecessarily link to KF5::ConfigCore

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure

Differential Revision: https://phabricator.kde.org/D1862