Remove the unused flags extension

It can always be brought back if we find an actual use case for it, but for now, it shouldn't be in the standard distribution given that it has no users.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143044153

java/google/registry/model/domain/DomainApplication.java

@@ -148,8 +148,7 @@ public class DomainApplication extends DomainBase {
       "dsig.xsd",
       "smd.xsd",
       "launch.xsd",
-      "allocate.xsd",
-      "flags.xsd");
+      "allocate.xsd");
 
   private static final XmlTransformer INPUT_TRANSFORMER =
       new XmlTransformer(SCHEMAS, EppInput.class);