src: remove unused includes in src/tracing

PR-URL: https://github.com/nodejs/node/pull/15682
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

src/tracing/agent.h

@@ -2,7 +2,6 @@
 #define SRC_TRACING_AGENT_H_
 
 #include "libplatform/v8-tracing.h"
-#include "node_platform.h"
 #include "uv.h"
 #include "v8.h"
 

src/tracing/trace_event.h

@@ -5,8 +5,6 @@
 #ifndef SRC_TRACING_TRACE_EVENT_H_
 #define SRC_TRACING_TRACE_EVENT_H_
 
-#include <stddef.h>
-
 #include "node_platform.h"
 #include "v8-platform.h"
 #include "trace_event_common.h"