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

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.

radicle-cli/src/commands/inbox.rs

@@ -124,7 +124,7 @@ impl Args for Options {
                         }
                     }
                 }
-                Long("repo") if mode.is_none() && op.is_some() => {
+                Long("repo") if mode.is_none() => {
                     let val = parser.value()?;
                     let repo = args::rid(&val)?;