chore: remove old and unused code

cli.go

@@ -108,8 +108,7 @@ Other:
 
 Boolean options accept explicit values, for example --cache=false.
 Command-line options override TOML settings. GH_REPO is used only when
---repo is absent. DIPLE_CONFIG selects a configuration file; GH_THREADS_CONFIG
-is retained as a migration fallback.
+--repo is absent. DIPLE_CONFIG selects a configuration file.
 
 Authentication uses GH_TOKEN or GITHUB_TOKEN when set, otherwise the active
 credential from 'gh auth login'. Run 'diple completion --help' for completion