Don't generate mix-ins when there aren't HTTP rules

From https://google.aip.dev/client-libraries/4234

> To be generated into a client library, a mixin service must be declared under [apis](https://github.com/googleapis/googleapis/blob/master/google/api/service.proto#L96) in a [google.api.Service](https://github.com/googleapis/googleapis/blob/master/google/api/service.proto). Furthermore, only the RPCs with [google.api.http](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto) bindings declared in the [http](https://github.com/googleapis/googleapis/blob/master/google/api/service.proto#L124) configuration of the same [google.api.Service](https://github.com/googleapis/googleapis/blob/master/google/api/service.proto) can be generated (more details on these bindings in [AIP-127](https://google.aip.dev/127)). If a mixin service RPC does not have a [google.api.http](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto) rule declared in the [google.api.Service](https://github.com/googleapis/googleapis/blob/master/google/api/service.proto), it must not be generated.

I believe we generate a mixin property based only on `apis`.