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/onboarding/mod.rs

@@ -1,6 +0,0 @@
-mod auth;
-pub mod onboarding_screen;
-mod trust_directory;
-pub(crate) use auth::mark_url_hyperlink;
-pub use trust_directory::TrustDirectorySelection;
-mod welcome;