Remove code branches for building with Qt 6

With the 5 series branched, there is no more purpose for those variants. And as they confuse doxygen and result in docs not being generated e.g. for Kdelibs4Migration, it is more simple to remove those code variants then teaching doxygen to evaluate those macro conditions as wanted. Similar for other static code analyzers.

Question: main motivation here were the missing docs. Does it make sense though to remove any such Qt6 code variants, for consistency?

Update: Answer from self: Having done a few patches, it feels inconsistent to just remove some of the conditions. Better do a complete wipe, but only for all those repos where public headers have this.