Don't hardcode QT_QPA_PLATFORMTHEME

Forcing QT_QPA_PLATFORMTHEME for all platforms breaks some useful platform integrations that Qt has to offer. For example portals (flatpak integration) and native file picker support. Seeing the GTK file picker open from an Qt applications looks pretty bad especially on KDE. I hope this will be fixed in the Qt 6.2 port but for now I have this patch: