Merge CoreDOMImplementationImpl into DOMImplementationImpl to simplify code base a bit

Author: rbri

src/main/java/org/htmlunit/cyberneko/xerces/dom/CoreDOMImplementationImpl.java

@@ -570,7 +570,7 @@ public NodeList getElementsByTagName(final String tagname) {
     public DOMImplementation getImplementation() {
         // Currently implemented as a singleton, since it's hardcoded
         // information anyway.
-        return CoreDOMImplementationImpl.getDOMImplementation();
+        return DOMImplementationImpl.getDOMImplementation();
     }
 
     /**
@@ -968,8 +968,8 @@ public Entity createEntity(final String name) throws DOMException {
     @Override
     protected int getNodeNumber() {
         if (documentNumber_ == 0) {
-            final CoreDOMImplementationImpl cd =
-                    (CoreDOMImplementationImpl) CoreDOMImplementationImpl.getDOMImplementation();
+            final DOMImplementationImpl cd =
+                    (DOMImplementationImpl) DOMImplementationImpl.getDOMImplementation();
             documentNumber_ = cd.assignDocumentNumber();
         }
         return documentNumber_;