Purge direct OpenGL code path in Qt Quick 3D

The counterpart of QTBUG-79268. However, there is no direct dependency between the two: direct OpenGL usage could be removed in Quick3D without doing the same in Quick (although the opposite is not necessarily true). So there is a certain flexibility when it comes to the timing of doing this.

Some cons for doing this too early:

• can make it difficult to merge from 5.15
• difficult to verify if the output of some feature is like before or not (because cannot just run with QSG_RHI=0 anymore, rather one needs to run and compare against 5.15)