remove unncessary synchronization

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1855956 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/ooxml/util/DocumentHelper.java

@@ -182,7 +182,7 @@ public final class DocumentHelper {
     /**
      * Creates a new DOM Document
      */
-    public static synchronized Document createDocument() {
+    public static Document createDocument() {
         return documentBuilderSingleton.newDocument();
     }