Fix 13247 by removing unnecessary judgment from method Serialize of DemoConsole.DesignerSerializationService (#13478)

<!-- Please read CONTRIBUTING.md before submitting a pull request -->

Fixes #13247

## Cause:

- This problem is due to the ToolStripItem not being serialized

Judgment `if (obj is Control)` was added to the `Serialize `method of
the `DemoConsole.DesignerSerializationService`, resulting in
`ToolStripItem` not being serialized
https://github.com/dotnet/winforms/blob/716bc129a4679a98394b2e2d24d1d1d3d4f54ddb/src/test/integration/DesignSurface/DesignSurfaceExt/DesignerSerializationServiceImpl.cs#L36-L37


[.netFramwork](https://devdiv.visualstudio.com/DevDiv/_git/VS?path=/src/vsip/Designer/Host/Serialization/CodeDomDesignerLoader.cs&version=GBmain&line=994&lineEnd=996&lineStartColumn=1&lineEndColumn=1&lineStyle=plain&_a=contents)
does not have this logic

## Proposed changes

- Removing unnecessary judgment `if (obj is Control)` from method
`Serialize` of `DemoConsole.DesignerSerializationService`

<!-- We are in TELL-MODE the following section must be completed -->

## Customer Impact

- The ToolStrip can be copy and paste normally 

## Regression? 

-  No

## Risk

- Minimal

<!-- end TELL-MODE -->


## Screenshots <!-- Remove this section if PR does not change UI -->

### Before

The subitem of the ToolStrip cannot be copied

![BeforeChangs](https://github.com/user-attachments/assets/4fc11858-d0f2-4387-8339-073fee242200)

### After
All of the content can be copied normally

![AfterChanges](https://github.com/user-attachments/assets/59efae48-52a3-48a3-b890-723e12f5e084)

## Test methodology <!-- How did you ensure quality? -->

- Manually


## Test environment(s) <!-- Remove any that don't apply -->

- .net 10.0.100-preview.5.25265.106


<!-- Mention language, UI scaling, or anything else that might be
relevant -->

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/dotnet/winforms/pull/13478)

Author: LeafShi1

src/test/integration/DesignSurface/DesignSurfaceExt/DesignerSerializationServiceImpl.cs

@@ -33,8 +33,7 @@ public object Serialize(ICollection objects)
         {
             foreach (object obj in objects)
             {
-                if (obj is Control)
-                    componentSerializationService.Serialize(serializationStore, obj);
+                componentSerializationService.Serialize(serializationStore, obj);
             }
 
             returnObject = serializationStore;