Remove code variants for building with Qt 6

Friedrich W. H. Kossebau requested to merge work/kossebau/dropqt6codevariants into kf5

With the 5 series branched, there is no more purpose for those variants. And they confuse at least doxygen (but surely also other static code analyzer).

Done for all modules which have such things in public headers. And consistently there to not make the codebase more strange.

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