src: reduce unnecessary includes

A forward declaration suffices and means that the tracing
agent header and its dependencies don’t need to be included
in nearly every Node.js file.

PR-URL: #21867
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>

src/env-inl.h

@@ -32,7 +32,6 @@
 #include "v8.h"
 #include "node_perf_common.h"
 #include "node_context_data.h"
-#include "tracing/agent.h"
 #include "node_worker.h"
 
 #include <stddef.h>

src/env.h

@@ -35,7 +35,6 @@
 #include "v8.h"
 #include "node.h"
 #include "node_http2_state.h"
-#include "tracing/agent.h"
 
 #include <list>
 #include <stdint.h>
@@ -55,6 +54,10 @@ namespace performance {
 class performance_state;
 }
 
+namespace tracing {
+class Agent;
+}
+
 namespace worker {
 class Worker;
 }