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/bin/md-events.rs

@@ -1,15 +0,0 @@
-use std::io::Read;
-use std::io::{self};
-
-fn main() {
-    let mut input = String::new();
-    if let Err(err) = io::stdin().read_to_string(&mut input) {
-        eprintln!("failed to read stdin: {err}");
-        std::process::exit(1);
-    }
-
-    let parser = pulldown_cmark::Parser::new(&input);
-    for event in parser {
-        println!("{event:?}");
-    }
-}