Revert "optimize(backend): optimize /api/posts/latest-reply"

This reverts commit 1e87e9252d.

backend/src/main/java/com/openisle/model/Post.java

@@ -67,10 +67,4 @@ public class Post {
     @Column
     private LocalDateTime pinnedAt;
 
-    @Column(nullable = false)
-    private long commentCount = 0;
-
-    @Column
-    private LocalDateTime lastReplyAt;
-
 }