Simplify the use of the fee extension a little

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133149148

java/google/registry/model/domain/fee06/FeeInfoCommandExtensionV06.java

@@ -25,15 +25,10 @@ import org.joda.money.CurrencyUnit;
 @XmlType(propOrder = {"currency", "command", "period"})
 public class FeeInfoCommandExtensionV06
     extends FeeQueryCommandExtensionItemImpl implements CommandExtension {
-  
+
   /** A three-character ISO4217 currency code. */
   CurrencyUnit currency;
 
-  @Override
-  public boolean isCurrencySupported() {
-    return true;
-  }
-
   @Override
   public CurrencyUnit getCurrency() {
     return currency;