Revert "all: Don't set AVClass.item_name to its default value"

Some callers assume that item_name is always set, so this may be
considered an API break.

This reverts commit 0c6203c97a.

libavcodec/vaapi_encode_mpeg2.c

@@ -682,6 +682,7 @@ static const FFCodecDefault vaapi_encode_mpeg2_defaults[] = {
 
 static const AVClass vaapi_encode_mpeg2_class = {
     .class_name = "mpeg2_vaapi",
+    .item_name  = av_default_item_name,
     .option     = vaapi_encode_mpeg2_options,
     .version    = LIBAVUTIL_VERSION_INT,
 };