avcodec/cbs_vp8: Remove empty flush callback

This callback is optional and should therefore only be set
if it actually does something.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/cbs_vp8.c

@@ -353,11 +353,6 @@ static int cbs_vp8_assemble_fragment(CodedBitstreamContext *ctx,
     return AVERROR_PATCHWELCOME;
 }
 
-static void cbs_vp8_flush(CodedBitstreamContext *ctx)
-{
-    // Do nothing.
-}
-
 static const CodedBitstreamUnitTypeDescriptor cbs_vp8_unit_types[] = {
     CBS_UNIT_TYPE_INTERNAL_REF(0, VP8RawFrame, data),
     CBS_UNIT_TYPE_END_OF_LIST,
@@ -374,7 +369,5 @@ const CodedBitstreamType ff_cbs_type_vp8 = {
     .read_unit         = &cbs_vp8_read_unit,
     .write_unit        = &cbs_vp8_write_unit,
 
-    .flush             = &cbs_vp8_flush,
-
     .assemble_fragment = &cbs_vp8_assemble_fragment,
 };