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.

libavformat/flvenc.c

@@ -1105,6 +1105,7 @@ static const AVOption options[] = {
 
 static const AVClass flv_muxer_class = {
     .class_name = "flv muxer",
+    .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,
 };