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/selection_list.rs

@@ -1,46 +0,0 @@
-use crate::render::renderable::Renderable;
-use crate::render::renderable::RowRenderable;
-use ratatui::style::Style;
-use ratatui::style::Styled as _;
-use ratatui::style::Stylize as _;
-use ratatui::widgets::Paragraph;
-use ratatui::widgets::Wrap;
-use unicode_width::UnicodeWidthStr;
-
-pub(crate) fn selection_option_row(
-    index: usize,
-    label: String,
-    is_selected: bool,
-) -> Box<dyn Renderable> {
-    selection_option_row_with_dim(index, label, is_selected, /*dim*/ false)
-}
-
-pub(crate) fn selection_option_row_with_dim(
-    index: usize,
-    label: String,
-    is_selected: bool,
-    dim: bool,
-) -> Box<dyn Renderable> {
-    let prefix = if is_selected {
-        format!("› {}. ", index + 1)
-    } else {
-        format!("  {}. ", index + 1)
-    };
-    let style = if is_selected {
-        Style::default().cyan()
-    } else if dim {
-        Style::default().dim()
-    } else {
-        Style::default()
-    };
-    let prefix_width = UnicodeWidthStr::width(prefix.as_str()) as u16;
-    let mut row = RowRenderable::new();
-    row.push(prefix_width, prefix.set_style(style));
-    row.push(
-        u16::MAX,
-        Paragraph::new(label)
-            .style(style)
-            .wrap(Wrap { trim: false }),
-    );
-    row.into()
-}