Remove DRI2/OpenGL code paths from xcb plugin.

Remove DRI2/OpenGL code paths from xcb plugin.

These were used to test QtWayland, but there are other options available
for that now. The DRI2 code hasn't been maintained and doesn't even
compile at the moment, so let's just remove it to de-clutter the source.

Change-Id: I7db0f4db82348497b9f4d6c2dcf2e13f3ab14a76
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>