lavfi/buffersink: schedule removing the "ff" variants.

They are no longer needed since there is no incompatible
ABI version.

libavfilter/allfilters.c

@@ -183,8 +183,10 @@ void avfilter_register_all(void)
     REGISTER_FILTER(AMOVIE,         amovie,         avsrc);
     REGISTER_FILTER(MOVIE,          movie,          avsrc);
 
+#if FF_API_AVFILTERBUFFER
     REGISTER_FILTER_UNCONDITIONAL(vsink_ffbuffersink);
     REGISTER_FILTER_UNCONDITIONAL(asink_ffabuffersink);
+#endif
 
     /* those filters are part of public or internal API => registered
      * unconditionally */

libavfilter/buffersink.c

@@ -471,6 +471,7 @@ static int asink_query_formats(AVFilterContext *ctx)
     return 0;
 }
 
+#if FF_API_AVFILTERBUFFER
 static const AVFilterPad ffbuffersink_inputs[] = {
     {
         .name      = "default",
@@ -511,6 +512,7 @@ AVFilter avfilter_asink_ffabuffersink = {
     .inputs        = ffabuffersink_inputs,
     .outputs       = NULL,
 };
+#endif /* FF_API_AVFILTERBUFFER */
 
 static const AVFilterPad avfilter_vsink_buffer_inputs[] = {
     {