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/FeeCheckCommandExtensionItemV06.java

@@ -29,7 +29,7 @@ public class FeeCheckCommandExtensionItemV06
   String name;
 
   CurrencyUnit currency;
-  
+
   @Override
   public boolean isDomainNameSupported() {
     return true;
@@ -40,11 +40,6 @@ public class FeeCheckCommandExtensionItemV06
     return name;
   }
 
-  @Override
-  public boolean isCurrencySupported() {
-    return true;
-  }
-
   @Override
   public CurrencyUnit getCurrency() {
     return currency;

java/google/registry/model/domain/fee06/FeeCheckCommandExtensionV06.java

@@ -32,18 +32,13 @@ public class FeeCheckCommandExtensionV06 extends ImmutableObject
     implements FeeCheckCommandExtension<
         FeeCheckCommandExtensionItemV06,
         FeeCheckResponseExtensionV06> {
-  
+
   @XmlElement(name = "domain")
   Set<FeeCheckCommandExtensionItemV06> items;
 
-  @Override
-  public boolean isCurrencySupported() {
-    return false;
-  }
-
   @Override
   public CurrencyUnit getCurrency() {
-    throw new UnsupportedOperationException("Currency not supported");
+    return null;  // This version of the fee extension doesn't specify a top-level currency.
   }
 
   @Override

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;