Eliminate unnecessary merge()

Fix GH-3401

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -195,7 +195,7 @@ public void delete(T entity) {
 			return;
 		}
 
-		entityManager.remove(entityManager.contains(entity) ? entity : entityManager.merge(entity));
+		entityManager.remove(existing);
 	}
 
 	@Override