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

Unnecessary since acf63d5350adeae551d412db699f8ca03f7e76b9;
also avoids relocations.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/opt_common.c

@@ -1069,7 +1069,6 @@ int opt_cpucount(void *optctx, const char *opt, const char *arg)
     };
     static const AVClass class = {
         .class_name = "cpucount",
-        .item_name  = av_default_item_name,
         .option     = opts,
         .version    = LIBAVUTIL_VERSION_INT,
     };