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;