qt: do not use qt opengl module anymore

Not all `QOpenGL` stuff are in the Qt OpenGL
module. It appears that `QOpenGLFrameBufferObject`
was the only class provided by Qt OpenGL and
the rest are provided within Qt GUI.

modules/gui/qt/qt6.pro

@@ -18,7 +18,6 @@ QTPLUGIN += qwindowsvistastyle
 }
 
 linux {
-QT += opengl # compositor_x11 requires Qt OpenGL
 QTPLUGIN += qxcb-glx-integration qxcb-egl-integration qxcb qwayland-generic qwayland-egl qgtk3 qxdgdesktopportal xdg-shell
 }