MINOR fix to deleteSubjectMode API (#4005)

Author: rayokota

core/src/main/java/io/confluent/kafka/schemaregistry/storage/SchemaRegistry.java

@@ -263,8 +263,8 @@ Set<String> subjects(String subject, boolean lookupDeletedSubjects) throws
   void deleteSubjectMode(String subject) throws SchemaRegistryStoreException,
           OperationNotPermittedException;
 
-  void deleteSubjectMode(String subject, boolean recursive) throws SchemaRegistryStoreException,
-          OperationNotPermittedException;
+  default void deleteSubjectMode(String subject, boolean recursive)
+      throws SchemaRegistryStoreException, OperationNotPermittedException {}
 
   RuleSetHandler getRuleSetHandler();