Remove sync

This is always called after creating a message. This does not appear to
be called again so I don't really see the need for timestampSet.
If really needed, using an AtomicLong would probably be better (with
compareAndSet(0, time)).
Found by coverity.

java/org/apache/catalina/ha/session/SessionMessageImpl.java

@@ -112,14 +112,13 @@ public String getSessionID() {
 
     /**
      * set message send time but only the first setting works (one shot)
+     * @param time the timestamp
      */
     @Override
     public void setTimestamp(long time) {
-        synchronized (this) {
-            if (!timestampSet) {
-                serializationTimestamp = time;
-                timestampSet = true;
-            }
+        if (!timestampSet) {
+            serializationTimestamp = time;
+            timestampSet = true;
         }
     }