Dynamic GL: remove exporting symbols

Remove the opengl proxy for now. Later it will either be moved into
a separate library or replaced by a QOpenGLFunctions-based approach.

This means that the -opengl dynamic configuration is not usable
for the time being. The rest of the enablers remain in place.

The convenience function QOpenGLFunctions::isES() is now moved to
QOpenGLContext and is changed to check the renderable type. This is
extremely useful since besides supporting dynamic GL it solves also
the problem of GL_ARB_ES2_compatibility (i.e. it triggers the real ES
path when creating an ES-compatible context with a desktop OpenGL
implementation).

Task-number: QTBUG-36483
Task-number: QTBUG-37172
Change-Id: I045be3fc16e9043e1528cf48e6bf0903da4fa7ca
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>