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

[jskeet]

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

To be generated into a client library, a mixin service must be declared under apis in a google.api.Service. Furthermore, only the RPCs with google.api.http bindings declared in the http configuration of the same google.api.Service can be generated (more details on these bindings in AIP-127). If a mixin service RPC does not have a google.api.http rule declared in the google.api.Service, it must not be generated.

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