tracing: Remove duplicate struct declaration in trace_events.h

struct trace_array is declared twice. One has been declared
at forward declaration. Remove the duplicate.

Link: https://lkml.kernel.org/r/20210330034056.2266969-1-wanjiabing@vivo.com

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/trace_events.h

@@ -404,7 +404,6 @@ trace_get_fields(struct trace_event_call *event_call)
 	return event_call->class->get_fields(event_call);
 }
 
-struct trace_array;
 struct trace_subsystem_dir;
 
 enum {