Remove override-build-system qt6
This was used in experimental BuildSystemQt6 class, which is not needed. This class only set the qt-install-dir to install-dir before calling prepare_module_build_environment(). This is not needed, because later there is a check if they are different and only if they are, the qt-install-dir specific variables are set. But for qt6-set and qt5-set the install-dir is set from qt-install-dir (as it must):
install-dir ${qt-install-dir}
And so, they never will be different.