fix service-script failing on check-spec

Hi,

On [this project](https://github.com/cldwalker/datomic-box), I'm trying to use datomic-crate with beta.10 and it fails while trying to use [service-script](https://github.com/rstradling/datomic-crate/blob/master/src/pallet/crate/datomic.clj#L79) with this error:

```
ExceptionInfo Invalid remote-file-arguments: Path [:service-impl] was not specified in the schema.  clojure.core/ex-info (core.clj:4327)
```

I traced this to `service-script` and it seems that a `check-spec` was recently added. This pull removes an extraneous key that fails the check.

Thanks for the great work you all are doing!