TrafficPolicy clean up

[npolshakova]

Now that agentgateway is using AgentgatewayPolicy instead of TrafficPolicy, clean up unused fields such as:

• https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L61
• References to agentgateway data plane only such as https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L215
• References to agentgateway data plane only such as https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L359
• https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L40
• https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L121
• https://github.com/kgateway-dev/kgateway/blob/main/api/v1alpha1/traffic_policy_types.go#L206