perf metric: Remove unused struct metric_ref_node

After commit 46bdc0bf8d21 ("perf metric: Simplify metric_refs
calculation"), no one use struct metric_ref_node, so remove it.

Signed-off-by: Yuan Can <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/linux-perf-users/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/metricgroup.c

···
 	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.