st/mesa: Drop dead 'passthrough_fs' field.

Dead since 2015 (commit 5142564734).

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_context.h

@@ -278,8 +278,6 @@ struct st_context
    /** for drawing with st_util_vertex */
    struct pipe_vertex_element util_velems[3];
 
-   void *passthrough_fs;  /**< simple pass-through frag shader */
-
    enum pipe_texture_target internal_target;
 
    struct cso_context *cso_context;