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/styles.md

@@ -1,21 +0,0 @@
-# Headers, primary, and secondary text
-
-- **Headers:** Use `bold`. For markdown with various header levels, leave in the `#` signs.
-- **Primary text:** Default.
-- **Secondary text:** Use `dim`.
-
-# Foreground colors
-
-- **Default:** Most of the time, just use the default foreground color. `reset` can help get it back.
-- **User input tips, selection, and status indicators:** Use ANSI `cyan`.
-- **Success and additions:** Use ANSI `green`.
-- **Errors, failures and deletions:** Use ANSI `red`.
-- **Codex:** Use ANSI `magenta`.
-
-# Avoid
-
-- Avoid custom colors because there's no guarantee that they'll contrast well or look good in various terminal color themes. (`shimmer.rs` is an exception that works well because we take the default colors and just adjust their levels.)
-- Avoid ANSI `black` & `white` as foreground colors because the default terminal theme color will do a better job. (Use `reset` if you need to in order to get those.) The exception is if you need contrast rendering over a manually colored background.
-- Avoid ANSI `blue` and `yellow` because for now the style guide doesn't use them. Prefer a foreground color mentioned above.
-
-(There are some rules to try to catch this in `clippy.toml`.)