Bug#850705: libqt5gui5: enabling software rendering in 90qt5-opengl

[Александр Волков]

Sorry, the software renderer is built-in in Qt 5.8.
So it makes sense to package qtdeclarative-render2d-opensource-src only 
for Qt 5.7.
And for Qt 5.8 you can remove 90qt5-opengl.