[Tracing] Remove unused variable kRuntimeCallStatsTracingEnabled.

kRuntimeCallStatsTracingEnabled was used as a global flag for runtime stats in
tracing, now it is no longer used.

TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2780293002
Cr-Commit-Position: refs/heads/master@{#44249}

src/tracing/trace-event.h

@@ -275,8 +275,6 @@ const int kZeroNumArgs = 0;
 const decltype(nullptr) kGlobalScope = nullptr;
 const uint64_t kNoId = 0;
 
-extern base::Atomic32 kRuntimeCallStatsTracingEnabled;
-
 class TraceEventHelper {
  public:
   static v8::Platform* GetCurrentPlatform();