Remove regtype extension since we won't be using it

If we do end up needing it we can simply revert this commit.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127094676

javatests/google/registry/flows/domain/DomainCheckFlowTest.java

@@ -497,16 +497,6 @@ public class DomainCheckFlowTest
     runFlow();
   }
 
-  @Test
-  public void testSuccess_regTypeExtensionValidates() throws Exception {
-    setEppInput("domain_check_regtype.xml");
-    persistActiveDomain("example1.tld");
-    doCheckTest(
-        create(false, "example1.tld", "In use"),
-        create(true, "example2.tld", null),
-        create(true, "example3.tld", null));
-  }
-
   @Test
   public void testSuccess_eapFeeCheck() throws Exception {
     clock.setTo(DateTime.parse("2010-01-01T10:00:00Z"));