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.

libavfilter/framesync.h

@@ -320,6 +320,7 @@ const AVClass *ff_framesync_child_class_iterate(void **iter);
 #define FRAMESYNC_DEFINE_PURE_CLASS(name, desc, func_prefix, options) \
 static const AVClass name##_class = {                                 \
     .class_name          = desc,                                      \
+    .item_name           = av_default_item_name,                      \
     .option              = options,                                   \
     .version             = LIBAVUTIL_VERSION_INT,                     \
     .category            = AV_CLASS_CATEGORY_FILTER,                  \