Clean up legacy configmodule querying

In KWin we are allowed to break compat and here all first party consumers
have already been ported for quite a while.

Also the querying is very inefficient, because for every plugin where the
X-KDE-ConfigModule property is not defined all available KCMs have to be opened and
their metadata parsed.

This way we also get rid of the deprecation warning for KPluginMetaData::readStringList.