libperf: Get rid of attr.id field

Now there's no in-tree user of the field.  To remove the possible bug
later, let's get rid of the 'id' field and add a comment for that.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230825152552.112913-4-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: namhyung

tools/lib/perf/include/perf/event.h

@@ -148,7 +148,13 @@ struct perf_record_switch {
 struct perf_record_header_attr {
 	struct perf_event_header header;
 	struct perf_event_attr	 attr;
-	__u64			 id[];
+	/*
+	 * Array of u64 id follows here but we cannot use a flexible array
+	 * because size of attr in the data can be different then current
+	 * version.  Please use perf_record_header_attr_id() below.
+	 *
+	 * __u64		 id[];  // do not use this
+	 */
 };
 
 /* Returns the pointer to id array based on the actual attr size. */