fix: unable to modify the publish time of post in the uc (#5596)

#### What type of PR is this?

/kind bug
/area core

#### What this PR does / why we need it:

修复在个人中心下，用户无法修改发布时间的问题。

#### How to test it?

需要测试两种情况：

1. 用户首次发布文章时，修改发布时间是否有效。
2. 用户后续编辑文章发布时间时是否生效。

#### Which issue(s) this PR fixes:

Fixes #5473 

#### Does this PR introduce a user-facing change?
```release-note
修复个人中心中用户无法修改文章发布时间的问题
```

application/src/main/java/run/halo/app/endpoint/uc/content/UcPostEndpoint.java

@@ -259,7 +259,6 @@ public class UcPostEndpoint implements CustomEndpoint {
                     var spec = post.getSpec();
                     spec.setOwner(oldSpec.getOwner());
                     spec.setPublish(oldSpec.getPublish());
-                    spec.setPublishTime(oldSpec.getPublishTime());
                     spec.setHeadSnapshot(oldSpec.getHeadSnapshot());
                     spec.setBaseSnapshot(oldSpec.getBaseSnapshot());
                     spec.setReleaseSnapshot(oldSpec.getReleaseSnapshot());