st_api: Remove st_context::is_visual_supported.

The callback is used by st/vega to check if a visual specifies the
depth/stencil format.  It forces st/vega to be loaded by st/egl to
perform the check.  As noted in EGL spec, the depth/stencil format of a
visual should not affect OpenVG.  It should be better to ignore the
field and always allocate the depth/stencil texture.

src/gallium/include/state_tracker/st_api.h

@@ -368,12 +368,6 @@ struct st_api
     */
    st_proc_t (*get_proc_address)(struct st_api *stapi, const char *procname);
 
-   /**
-    * Return true if the visual is supported by the state tracker.
-    */
-   boolean (*is_visual_supported)(struct st_api *stapi,
-                                  const struct st_visual *visual);
-
    /**
     * Create a rendering context.
     */