Resolve warnings in the Hibernate log (#542)

core/src/main/java/google/registry/schema/tld/PremiumEntry.java

@@ -14,6 +14,7 @@
 
 package google.registry.schema.tld;
 
+import google.registry.model.ImmutableObject;
 import java.io.Serializable;
 import java.math.BigDecimal;
 import javax.persistence.Column;
@@ -26,7 +27,7 @@ import javax.persistence.Id;
  * <p>These are not persisted directly, but rather, using {@link PremiumList#getLabelsToPrices()}.
  */
 @Entity
-public class PremiumEntry implements Serializable {
+public class PremiumEntry extends ImmutableObject implements Serializable {
 
   @Id
   @Column(nullable = false)

core/src/main/resources/META-INF/orm.xml

@@ -4,15 +4,10 @@
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence/orm
           http://xmlns.jcp.org/xml/ns/persistence/orm_2_2.xsd"
     version="2.2">
-  <embeddable class="org.joda.money.Money" access="FIELD">
-    <attributes>
-      <embedded name="money" access="FIELD"/>
-    </attributes>
-  </embeddable>
+  <embeddable class="org.joda.money.Money" access="FIELD" />
   <embeddable class="org.joda.money.BigMoney" access="FIELD">
     <attributes>
       <basic name="amount" access="FIELD"/>
-      <basic name="currency" access="FIELD"/>
     </attributes>
   </embeddable>
 </entity-mappings>