fixes [`RawChip` doesn't use `ChipThemeData.showCheckmark` value](https://github.com/flutter/flutter/issues/119163)

### Description

`RawChip.showCheckmark` is nullable yet the constructor falsely assigns a default which breaks `ChipTheme` support. This PR removes the falsely assigned default value.

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(useMaterial3: true,
        chipTheme: const ChipThemeData(
          showCheckmark: false,
        )
      ),
      home: const Example(),
    );
  }
}

class Example extends StatelessWidget {
  const Example({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample'),
      ),
      body: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: <Widget>[
            const RawChip(
              selected: true,
              label: Text('RawChip'),
            ),
            FilterChip(
              selected: true,
              label: const Text('RawChip'), onSelected: (bool value) {  },
            ),
          ],
        ),
      ),
    );
  }
}

``` 
	
</details>

### Before
![before](https://github.com/flutter/flutter/assets/48603081/c8050c28-d988-4c72-8e0a-6455aa02d119)

### After

![after](https://github.com/flutter/flutter/assets/48603081/d5e83e81-6c12-4594-a2fd-8f113d6c9b54)