Now that qtquickcontrols2 has been merged into qtdeclarative,
we should make it obvious that this repo should no longer be
used, by preventing it from being built.

Task-number: QTBUG-95173
Pick-to: 6.2
Change-Id: I95bd6a214f3d75a865ab163ee0a1f9ffbeb7a051
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>