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/test_support.rs

@@ -1,28 +0,0 @@
-use codex_utils_absolute_path::AbsolutePathBuf;
-use std::path::Path;
-use std::path::PathBuf;
-
-pub(crate) trait PathExt {
-    fn abs(&self) -> AbsolutePathBuf;
-}
-
-impl PathExt for Path {
-    fn abs(&self) -> AbsolutePathBuf {
-        AbsolutePathBuf::try_from(self.to_path_buf())
-            .unwrap_or_else(|_| panic!("path should already be absolute"))
-    }
-}
-
-pub(crate) trait PathBufExt {
-    fn abs(&self) -> AbsolutePathBuf;
-}
-
-impl PathBufExt for PathBuf {
-    fn abs(&self) -> AbsolutePathBuf {
-        self.as_path().abs()
-    }
-}
-
-pub(crate) fn test_path_display(path: &str) -> String {
-    Path::new(path).abs().display().to_string()
-}