Remove unused fields from JsonSchemaVersion (#642)

Author: valfirst

src/main/java/com/networknt/schema/JsonSchemaVersion.java

@@ -4,8 +4,6 @@
 import java.util.List;
 
 public abstract class JsonSchemaVersion {
-    protected static String URI;
-    protected static String ID;
     public static final List<Format> BUILTIN_FORMATS = new ArrayList<Format>(JsonMetaSchema.COMMON_BUILTIN_FORMATS);
     static {
         // add version specific formats here.