worker: remove unused blank identifier
worker: remove unused blank identifier

the use of such blank identifier there was mostly for getting early
feedback about the compilation of `backend` back then when it wasn't
integrated into the whole `workercmd` package.

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse