refactor(app): drop an opener nothing has called since the first commit

`open_config_file` has been `#[allow(dead_code)]` since
22e1ab16 — the initial commit — with nothing to say why. It is not
staged for anything: it was dead when it arrived.

It also hand-rolls a platform opener, picking between `open`, `explorer`
and `xdg-open` itself, while this file already reaches for
`cx.open_with_system` two thousand lines up and `terminal::view::
open_file_path` exists for the same job. Whoever wires an "open
config.json" affordance will write one line, not these twenty, and will
get the error reporting the file-link opener already has.

The other `allow(dead_code)` nearby is the counter-example and stays:
`palette::CheckoutBranch` says in its doc what it is waiting for —
"nothing emits it until the picker can list refs" — which is a parked
feature rather than a forgotten one. An allow that explains itself is
worth keeping; this one never did.

src/ui/app.rs

@@ -6542,26 +6542,6 @@ impl Tty7App {
         crate::ui::keymap::rebind(cx);
         cx.notify();
     }
-
-    #[allow(dead_code)]
-    pub(crate) fn open_config_file(&self, cx: &Context<Self>) {
-        let Some(path) = crate::core::config::config_path("config.json") else {
-            return;
-        };
-        if !path.exists() {
-            cx.global::<Config>().save();
-        }
-        let opener = if cfg!(target_os = "macos") {
-            "open"
-        } else if cfg!(windows) {
-            "explorer"
-        } else {
-            "xdg-open"
-        };
-        if let Err(e) = std::process::Command::new(opener).arg(&path).spawn() {
-            log::warn!("failed to open {}: {e}", path.display());
-        }
-    }
 }
 
 #[cfg(test)]