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/tests/suite/status_indicator.rs

@@ -1,24 +0,0 @@
-//! Regression test: ensure that `StatusIndicatorWidget` sanitises ANSI escape
-//! sequences so that no raw `\x1b` bytes are written into the backing
-//! buffer.  Rendering logic is tricky to unit‑test end‑to‑end, therefore we
-//! verify the *public* contract of `ansi_escape_line()` which the widget now
-//! relies on.
-
-use codex_ansi_escape::ansi_escape_line;
-
-#[test]
-fn ansi_escape_line_strips_escape_sequences() {
-    let text_in_ansi_red = "\x1b[31mRED\x1b[0m";
-
-    // The returned line must contain three printable glyphs and **no** raw
-    // escape bytes.
-    let line = ansi_escape_line(text_in_ansi_red);
-
-    let combined: String = line
-        .spans
-        .iter()
-        .map(|span| span.content.to_string())
-        .collect();
-
-    assert_eq!(combined, "RED");
-}