cli: remove op check for `--repo` in `rad inbox` — heartwood

As a response to Issue # c4134924cf2ce84444c6f7235a27651ad49311b5. A no-op command in rad inbox defaults to show, but the inbox argument parser only permits –repo in instances where the operation is explicity declared.

Here, the check for an explicit operation is removed so one is never required.

                         }
                     }
                 }
                 Long("repo") if mode.is_none() && op.is_some() => {
                 Long("repo") if mode.is_none() => {
                     let val = parser.value()?;
                     let repo = args::rid(&val)?;