Revert "util: Check whether discovered powershell is actually executable" (#43247)

Reverts zed-industries/zed#43044
Closes https://github.com/zed-industries/zed/issues/43224

This slows down startup on windows significantly

Release Notes:

- Fixed slow startup on Windows

crates/askpass/src/askpass.rs

@@ -250,7 +250,6 @@ impl PasswordProxy {
             .await
             .with_context(|| format!("creating askpass script at {askpass_script_path:?}"))?;
         make_file_executable(&askpass_script_path).await?;
-        // todo(shell): There might be no powershell on the system
         #[cfg(target_os = "windows")]
         let askpass_helper = format!(
             "powershell.exe -ExecutionPolicy Bypass -File {}",