glx: remove miss leading comment

There was a comment implying that if a rgba config has been requested
but no suitable config was found then we would automatically fall back
to an rgb config instead. Actually if no rgba visual is found we simply
fail without any automatic fall back because Cogl is not in a good
position to judge if automatic fall backs are acceptable for higher
level apis such as clutter.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/winsys/cogl-winsys-glx.c

@@ -559,8 +559,6 @@ find_fbconfig (CoglDisplay *display,
             }
         }
 
-      /* If we make it here then we didn't find an RGBA config so
-         we'll fall back to using an RGB config */
       g_set_error (error, COGL_WINSYS_ERROR,
                    COGL_WINSYS_ERROR_CREATE_CONTEXT,
                    "Unable to find fbconfig with rgba visual");