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/Cargo.toml

@@ -49,7 +49,6 @@ members = [
     "sandboxing",
     "stdio-to-uds",
     "otel",
-    "tui",
     "tui_app_server",
     "tools",
     "v8-poc",
@@ -150,8 +149,7 @@ codex-state = { path = "state" }
 codex-stdio-to-uds = { path = "stdio-to-uds" }
 codex-terminal-detection = { path = "terminal-detection" }
 codex-tools = { path = "tools" }
-codex-tui = { path = "tui" }
-codex-tui-app-server = { path = "tui_app_server" }
+codex-tui = { path = "tui_app_server" }
 codex-v8-poc = { path = "v8-poc" }
 codex-utils-absolute-path = { path = "utils/absolute-path" }
 codex-utils-approval-presets = { path = "utils/approval-presets" }