lavfi/avfilter: remove now unused {src,cur,out}_buf.

This was supposed to be removed in 565e4993, but it was delayed due to
the port of the remaining filters. These buffer ref pointers are not
used anymore, thus dropped.

libavfilter/avfilter.h

@@ -583,51 +583,6 @@ struct AVFilterLink {
         AVLINK_INIT             ///< complete
     } init_state;
 
-    /**
-     * The buffer reference currently being sent across the link by the source
-     * filter. This is used internally by the filter system to allow
-     * automatic copying of buffers which do not have sufficient permissions
-     * for the destination. This should not be accessed directly by the
-     * filters.
-     */
-    AVFilterBufferRef *src_buf;
-
-    /**
-     * The buffer reference to the frame sent across the link by the
-     * source filter, which is read by the destination filter. It is
-     * automatically set up by ff_start_frame().
-     *
-     * Depending on the permissions, it may either be the same as
-     * src_buf or an automatic copy of it.
-     *
-     * It is automatically freed by the filter system when calling
-     * ff_end_frame(). In case you save the buffer reference
-     * internally (e.g. if you cache it for later reuse), or give it
-     * away (e.g. if you pass the reference to the next filter) it
-     * must be set to NULL before calling ff_end_frame().
-     */
-    AVFilterBufferRef *cur_buf;
-
-    /**
-     * The buffer reference to the frame which is sent to output by
-     * the source filter.
-     *
-     * If no start_frame callback is defined on the link destination pad,
-     * ff_start_frame() will automatically request a new buffer on the
-     * first output link of the destination filter. The reference to
-     * the buffer so obtained is stored in the out_buf field on the
-     * output link.
-     *
-     * It can also be set by the filter code in case the filter needs
-     * to access the output buffer later. For example the filter code
-     * may set it in a custom start_frame, and access it in
-     * draw_slice.
-     *
-     * It is automatically freed by the filter system in
-     * ff_end_frame().
-     */
-    AVFilterBufferRef *out_buf;
-
     struct AVFilterPool *pool;
 
     /**