Reverting #1971 as it was causing latency issues in test canaries

Author: John Viegas

.changes/next-release/bugfix-AWSSDKforJavav2-da92594.json

@@ -0,0 +1,6 @@
+{
+    "category": "AWS SDK for Java v2", 
+    "contributor": "", 
+    "type": "bugfix", 
+    "description": "Reverting synchronization of eventsToDeliver Object in onComplete() [PR#1971](https://github.com/aws/aws-sdk-java-v2/pull/1971) since it was causing latency."
+}

core/aws-core/src/main/java/software/amazon/awssdk/awscore/eventstream/EventStreamAsyncResponseTransformer.java

@@ -394,9 +394,7 @@ public void onError(Throwable throwable) {
         @Override
         public void onComplete() {
             // Add the special on complete event to signal drainEvents to complete the subscriber
-            synchronized (eventsToDeliver) {
-                eventsToDeliver.add(ON_COMPLETE_EVENT);
-            }
+            eventsToDeliver.add(ON_COMPLETE_EVENT);
             drainEventsIfNotAlready();
             transformFuture.complete(null);
         }