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

javatests/google/registry/tools/UpdateRegistrarCommandTest.java

@@ -28,14 +28,12 @@ import com.beust.jcommander.ParameterException;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
-import google.registry.model.billing.RegistrarBillingEntry;
 import google.registry.model.registrar.Registrar;
 import google.registry.model.registrar.Registrar.BillingMethod;
 import google.registry.model.registrar.Registrar.State;
 import google.registry.model.registrar.Registrar.Type;
 import google.registry.util.CidrAddressBlock;
 import org.joda.money.CurrencyUnit;
-import org.joda.money.Money;
 import org.joda.time.DateTime;
 import org.junit.Test;
 
@@ -269,30 +267,6 @@ public class UpdateRegistrarCommandTest extends CommandTestCase<UpdateRegistrarC
         .isEqualTo(BillingMethod.BRAINTREE);
   }
 
-  @Test
-  public void testFailure_changeBillingMethodWhenBalanceIsNonZero() {
-    createTlds("xn--q9jyb4c");
-    Registrar registrar = loadRegistrar("NewRegistrar");
-    persistResource(
-        new RegistrarBillingEntry.Builder()
-            .setPrevious(null)
-            .setParent(registrar)
-            .setCreated(DateTime.parse("1984-12-18TZ"))
-            .setDescription("USD Invoice for December")
-            .setAmount(Money.parse("USD 10.00"))
-            .build());
-    assertThat(registrar.getBillingMethod()).isEqualTo(BillingMethod.EXTERNAL);
-    IllegalStateException thrown =
-        assertThrows(
-            IllegalStateException.class,
-            () -> runCommand("--billing_method=braintree", "--force", "NewRegistrar"));
-    assertThat(thrown)
-        .hasMessageThat()
-        .isEqualTo(
-            "Refusing to change billing method on Registrar 'NewRegistrar' from EXTERNAL to "
-                + "BRAINTREE because current balance is non-zero: {USD=USD 10.00}");
-  }
-
   @Test
   public void testSuccess_streetAddress() throws Exception {
     runCommand("--street=\"1234 Main St\"", "--street \"4th Floor\"", "--street \"Suite 1\"",