Use O3 causes warnings when combined with O2, so just remove it.
Partially revert commit 11111c5a7d
Change-Id: Ifbf6e024e35933ecc3610d6efc3423589dab9a38
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>