Fix unreachable code warning

The return NULL statement at the bottom of
wxAuiXmlHandler::DoCreateResource() is never reached because the catch-all
else branch above it always returns. Fix by removing the bottom return
statement (same as how other *XmlHandler::DoCreateResource() methods
handle this, and not by changing the else into an else if).

src/xrc/xh_aui.cpp

@@ -289,8 +289,6 @@ wxObject *wxAuiXmlHandler::DoCreateResource()
 
         return anb;
     }
-
-    return NULL;
 }
 
 bool wxAuiXmlHandler::CanHandle( wxXmlNode *node )