OpenGL: Remove API for stubbed Geometry shader support in gui/opengl

OpenGL: Remove API for stubbed Geometry shader support in gui/opengl

There is a pending patch to reinstate this for Qt 5.1 (the patch
introduces new API top QOpenGLContext). I will fold the revert of this
patch into:

https://codereview.qt-project.org/#change,31231

Note that QGLShaderProgram still supports Geometry shaders so source
compatibility with Qt 4.8 is maintained.

Change-Id: I53faedd4e4a2a6c2a9765afdc1bee6df5ae2f711
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>