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.

docs/tui-stream-chunking-tuning.md

@@ -6,8 +6,8 @@ changing the underlying policy shape.
 ## Scope
 
 Use this guide when adjusting queue-pressure thresholds and hysteresis windows in
-`codex-rs/tui/src/streaming/chunking.rs`, and baseline commit cadence in
-`codex-rs/tui/src/app.rs`.
+`codex-rs/tui_app_server/src/streaming/chunking.rs`, and baseline commit cadence in
+`codex-rs/tui_app_server/src/app.rs`.
 
 This guide is about tuning behavior, not redesigning the policy.