Remove openglextensions module in Qt 6

https://code.qt.io/cgit/qt/qtbase.git/tree/src/openglextensions is not something we want to ship and support as part of Qt in the future.

Its usage has been discouraged internally in Qt (as there are often better alternatives, such as QOpenGLExtraFunctions, for the functionality needed by Qt Quick or Qt Quick 3D)

With the new RHI-based graphics stack (where the OpenGL backend of QRhi does not rely on openglextensions either), there will be no direct usages of OpenGL outside of QtGui. (and where it is still desired, resolving extensions is always possible in many different ways, the openglextension module was just one possible convenience)

For applications, the assumption is that this module is rarely used. There are also packaging issues: it turns out that the debug version of the static library has not been shipped (on some platforms at least). This makes wide spread usage unlikely...