SPDX/reuse does not feature any generic 'public domain' license
identifier. So far we claimed CC0, which is however not entirely correct.
Now that we have LICENSES directory, let's correctly claim this as
a specific license.
Also, remove the LGPL in-code claim. The adaptations for Qt code is
minor so it doesn't make much sense to claim a different license.
Pick-to: 6.5
Change-Id: I4e943a45baae192b54c38184e8eb85fc6d4035e5
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
