mod: 사용하지 않는 메서드 삭제

Author: jjjh02

ontime-back/src/main/java/devkor/ontime_back/repository/PreparationUserRepository.java

@@ -34,6 +34,4 @@ public interface PreparationUserRepository extends JpaRepository<PreparationUser
     void clearNextPreparationByUserId(@Param("userId") Long userId);
 
     boolean existsByUser(User user);
-
-    List<PreparationUser> findByUser(User user);
 }