jobspec: the `Service.Id` field is never used but passes validation

The Nomad API service struct includes an [Id field](https://github.com/hashicorp/nomad/blob/2fc19c398d4261c40c996d76fea2ccd77f3086e6/api/services.go#L105) that is never used when [creating a Consul service registration ID](https://github.com/hashicorp/nomad/blob/4a1a14248a7546a7423597ed8259e85d551e8cc1/command/agent/consul/service_client.go#L1432-L1438). This field is not present within the [internal struct representation](https://github.com/hashicorp/nomad/blob/main/nomad/structs/services.go#L425-L470) of a service. Additionally, a job registers successfully when it includes a service that has an `id` parameter defined.

I therefore propose to remove the API field previously indicated, as well as update the jobspec parsing to error when a service definition includes an `id` parameter.

Start with api/services.go and nomad/structs/services.go, then trace the Consul registration path in command/agent/consul/service_client.go. Done means the unused API field is removed and jobspec parsing rejects service definitions containing an id parameter.

Written by the indexing model from the issue text.