Fixed thread local cleanup detected by IDEA.

Author: AlexeyKuznetsov-DD

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/instrumentation/java/concurrent/TPEHelper.java

@@ -101,7 +101,7 @@ public static AgentScope getAndClearThreadLocalScope(Runnable task) {
       return null;
     }
     AgentScope scope = threadLocalScope.get();
-    threadLocalScope.set(null);
+    threadLocalScope.remove();
     return scope;
   }