package/qt5/qt5base: drop wrong optimization flag

In qmake.conf.in has been left 'QMAKE_CXXFLAGS_RELEASE += -O3' but this
leads to not use Buildroot CXXFLAGS when building in release
mode(without debugging symbols). So let's remove it to let Qt5 to follow
Buildroot optimization flags like other packages do.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>