perf metric: Remove unused struct metric_ref_node

After commit 46bdc0bf8d ("perf metric: Simplify metric_refs
calculation"), no one use struct metric_ref_node, so remove it.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/linux-perf-users/20220927013931.110475-4-yuancan@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/metricgroup.c

@@ -109,17 +109,6 @@ void metricgroup__rblist_exit(struct rblist *metric_events)
 	rblist__exit(metric_events);
 }
 
-/*
- * A node in the list of referenced metrics. metric_expr
- * is held as a convenience to avoid a search through the
- * metric list.
- */
-struct metric_ref_node {
-	const char *metric_name;
-	const char *metric_expr;
-	struct list_head list;
-};
-
 /**
  * The metric under construction. The data held here will be placed in a
  * metric_expr.