tracing: Remove unnecessary hist trigger struct field

hist_field.var_idx is completely unused, so remove it.

Link: http://lkml.kernel.org/r/d4e066c0f509f5f13ad3babc8c33ca6e7ddc439a.1545161087.git.tom.zanussi@linux.intel.comAcked-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -61,7 +61,6 @@ struct hist_field {
 	char				*system;
 	char				*event_name;
 	char				*name;
-	unsigned int			var_idx;
 	unsigned int			var_ref_idx;
 	bool                            read_once;
 };