ecs: unable to change networkMode back to the default

### Describe the bug

An EcsService defined like this:

```ts

const taskDefinition = new ecs.Ec2TaskDefinition(this, 'TaskDef', {
family: 'task-def',
taskRole: taskRole,
networkMode: ecs.NetworkMode.AWS_VPC,
});

const realtimeWsService = new ecs.Ec2Service(this, 'Service', {
serviceName: 'Service',
cluster: props.cluster,
taskDefinition,
capacityProviderStrategies: [gpucapacityProviderStrategy],
serviceConnectConfiguration: {
logDriver: new ecs.AwsLogDriver({
logGroup,
streamPrefix: 'serviceconnect',
}),
namespace: 'local',
// realtimews.local:5001
services: [{ portMappingName: 'rtws', dnsName: 'rtws' }],
},
});

this.lb = new elbv2.ApplicationLoadBalancer(this, 'LB', {
loadBalancerName: 'Ecs-RealtimeWs-ALB',
vpc: props.vpc,
internetFacing: true,
preserveHostHeader: true,
});

const listener = this.lb.addListener('PublicListener', {
port: 80,
open: true,
protocol: elbv2.ApplicationProtocol.HTTP,
});

// Attach ALB to ECS Service
listener.addTargets('ECS', {
targetGroupName: 'EcsWebsocketAlbTG',
port: 80,
protocol: elbv2.ApplicationProtocol.HTTP,
targets: [
realtimeWsService.loadBalancerTarget({
containerName: containerDef.containerName,
containerPort: containerDef.containerPort,
}),
],
});
```

This deploys and works perfectly fine. However, now, I want to change the approach and I want to remove the public ALB altogether and go back to the default `networkMode`.

```ts

const taskDefinition = new ecs.Ec2TaskDefinition(this, 'TaskDef', {
family: 'task-def',
taskRole: taskRole,
// networkMode: ecs.NetworkMode.AWS_VPC,
});

const realtimeWsService = new ecs.Ec2Service(this, 'Service', {
serviceName: 'Service',
cluster: props.cluster,
taskDefinition,
capacityProviderStrategies: [gpucapacityProviderStrategy],
serviceConnectConfiguration: {
logDriver: new ecs.AwsLogDriver({
logGroup,
streamPrefix: 'serviceconnect',
}),
namespace: 'local',
// realtimews.local:5001
services: [{ portMappingName: 'rtws', dnsName: 'rtws' }],
},
});

```

This bails out with:

> Service-Stack | 2/6 | 10:42:38 PM | UPDATE_FAILED | AWS::ECS::Service | Resource handler returned message: "Invalid request provided: UpdateService error: The target group arn:aws:elasticloadbalancing:us-west-2:679849022850:targetgroup/EcsWebsocketAlbTG/efbc21827edf62e8 does not exist. (Service: AmazonECS; Status Code: 400; Error Code: InvalidParameterException;

If you check the synchronised template, you'll see it is still referencing the TargetGroup, even though it's nowhere in the code.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

above

### Current Behavior

above

### Reproduction Steps

above

### Possible Solution

_No response_

### Additional Information/Context

```json
"devDependencies": {
"@biomejs/biome": "1.9.4",
"@types/babel__traverse": "^7.18.2",
"@types/js-yaml": "^4.0.5",
"@types/node": "^22.10.2",
"@typescript-eslint/eslint-plugin": "^8.18.0",
"@typescript-eslint/parser": "^8.18.0",
"aws-cdk": "^2.80.0",
"lefthook": "^1.10.1",
"typescript": "^5.6.3"
},
"dependencies": {
"aws-cdk": "^2.80.0",
"aws-cdk-lib": "^2.80.0",
"cdk-nag": "^2.28.179",
"cloudwatch-retention-setter": "^0.0.15",
"constructs": "^10.0.0",
"js-yaml": "^4.1.0",
"source-map-support": "^0.5.21"
}
```

### CDK CLI Version

2.177.0 (build b396961)

### Framework Version

_No response_

### Node.js Version

v22.12.0

### OS

MacOS

### Language

TypeScript

### Language Version

```
├── @biomejs/biome@1.9.4
├── @types/babel__traverse@7.20.6
├── @types/js-yaml@4.0.9
├── @types/node@22.10.10
├── @typescript-eslint/eslint-plugin@8.21.0
├── @typescript-eslint/parser@8.21.0
├── aws-cdk-lib@2.177.0
├── aws-cdk@2.177.0
├── cdk-nag@2.35.1
├── cloudwatch-retention-setter@0.0.15
├── constructs@10.4.2
├── js-yaml@4.1.0
├── lefthook@1.10.10
├── source-map-support@0.5.21
└── typescript@5.7.3
```

### Other information

_No response_

Reproduce the transition from an Ec2TaskDefinition using AWS_VPC and an ALB target to the default network mode, then inspect the synthesized template and ECS service update. Trace the Ec2Service and loadBalancerTarget handling; done means the removed target group is no longer referenced and the stack update succeeds.

Written by the indexing model from the issue text.