Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/src/app_event_sender.rs

@@ -1,28 +0,0 @@
-use tokio::sync::mpsc::UnboundedSender;
-
-use crate::app_event::AppEvent;
-use crate::session_log;
-
-#[derive(Clone, Debug)]
-pub(crate) struct AppEventSender {
-    pub app_event_tx: UnboundedSender<AppEvent>,
-}
-
-impl AppEventSender {
-    pub(crate) fn new(app_event_tx: UnboundedSender<AppEvent>) -> Self {
-        Self { app_event_tx }
-    }
-
-    /// Send an event to the app event channel. If it fails, we swallow the
-    /// error and log it.
-    pub(crate) fn send(&self, event: AppEvent) {
-        // Record inbound events for high-fidelity session replay.
-        // Avoid double-logging Ops; those are logged at the point of submission.
-        if !matches!(event, AppEvent::CodexOp(_)) {
-            session_log::log_inbound_app_event(&event);
-        }
-        if let Err(e) = self.app_event_tx.send(event) {
-            tracing::error!("failed to send event: {e}");
-        }
-    }
-}