Revert "su: allocate new pty (#1693)"

This reverts commit 935dc18faa.

userspace/ksud/src/cli.rs

@@ -1,6 +1,6 @@
 use anyhow::{Ok, Result};
 use clap::Parser;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 #[cfg(target_os = "android")]
 use android_logger::Config;
@@ -283,7 +283,7 @@ pub fn run() -> Result<()> {
 
     // the kernel executes su with argv[0] = "su" and replace it with us
     let arg0 = std::env::args().next().unwrap_or_default();
-    if Path::new(&arg0).file_name().and_then(|f| f.to_str()) == Some("su") {
+    if arg0 == "su" || arg0 == "/system/bin/su" {
         return crate::su::root_shell();
     }