xlib: remove dummy GLX_MESA_set_3dfx_mode implementation

The implementation is a simple 'return EGL_FALSE'. Stop pretending and
simply remove it.

Note: the removal of XMesa API is fine, since there hasn't been any
users for it in years.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/relnotes/17.4.0.html

@@ -61,7 +61,7 @@ TBD
 <h2>Changes</h2>
 
 <ul>
-TBD
+<li>Remove incomplete GLX_MESA_set_3dfx_mode from the Xlib libGL</li>
 </ul>
 
 </div>