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/vt100_live_commit.rs

@@ -1,45 +0,0 @@
-#![cfg(feature = "vt100-tests")]
-
-use crate::test_backend::VT100Backend;
-use ratatui::layout::Rect;
-use ratatui::text::Line;
-
-#[test]
-fn live_001_commit_on_overflow() {
-    let backend = VT100Backend::new(20, 6);
-    let mut term = match codex_tui::custom_terminal::Terminal::with_options(backend) {
-        Ok(t) => t,
-        Err(e) => panic!("failed to construct terminal: {e}"),
-    };
-    let area = Rect::new(0, 5, 20, 1);
-    term.set_viewport_area(area);
-
-    // Build 5 explicit rows at width 20.
-    let mut rb = codex_tui::live_wrap::RowBuilder::new(20);
-    rb.push_fragment("one\n");
-    rb.push_fragment("two\n");
-    rb.push_fragment("three\n");
-    rb.push_fragment("four\n");
-    rb.push_fragment("five\n");
-
-    // Keep the last 3 in the live ring; commit the first 2.
-    let commit_rows = rb.drain_commit_ready(3);
-    let lines: Vec<Line<'static>> = commit_rows.into_iter().map(|r| r.text.into()).collect();
-
-    codex_tui::insert_history::insert_history_lines(&mut term, lines)
-        .expect("Failed to insert history lines in test");
-
-    let screen = term.backend().vt100().screen();
-
-    // The words "one" and "two" should appear above the viewport.
-    let joined = screen.contents();
-    assert!(
-        joined.contains("one"),
-        "expected committed 'one' to be visible\n{joined}"
-    );
-    assert!(
-        joined.contains("two"),
-        "expected committed 'two' to be visible\n{joined}"
-    );
-    // The last three (three,four,five) remain in the live ring, not committed here.
-}