Remove not used package in service.tmpl

It looks like we don't need `credential.Provider` anymore, we can remove the import:

```go
"github.com/beyondstorage/go-storage/v4/pkg/credential"
```

## Steps

- [ ] Remove the import from `cmd/definitions/tmpl/service.tmpl`
- [ ] `make build` to trigger generation