Delete remnants of registrar-level credits

We never fully used this stuff but definitely no longer use it following our
recent billing refactor. It's confusing to retain all of these entities and
commands given that none of them are actually used by anything.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201978094

java/google/registry/model/EntityClasses.java

@@ -16,9 +16,6 @@ package google.registry.model;
 
 import com.google.common.collect.ImmutableSet;
 import google.registry.model.billing.BillingEvent;
-import google.registry.model.billing.RegistrarBillingEntry;
-import google.registry.model.billing.RegistrarCredit;
-import google.registry.model.billing.RegistrarCreditBalance;
 import google.registry.model.common.Cursor;
 import google.registry.model.common.EntityGroupRoot;
 import google.registry.model.common.GaeUserIdConverter;
@@ -103,10 +100,7 @@ public final class EntityClasses {
           PremiumList.PremiumListRevision.class,
           RdeRevision.class,
           Registrar.class,
-          RegistrarBillingEntry.class,
           RegistrarContact.class,
-          RegistrarCredit.class,
-          RegistrarCreditBalance.class,
           Registry.class,
           ReservedList.class,
           ServerSecret.class,