Also remove ImplWallpaper::operator==, that does not
make sense since members are unique

Change-Id: I69d32d91ba33691eb1f86e70ce3c53fa2761e34b
Reviewed-on: https://gerrit.libreoffice.org/31666Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>