Remove long-unused autoenv tests (#14436)

# Description

The `.nu-env` file feature was removed some time ago (probably in the
engine-q upgrade?). The tests, however, still remained as dead-code, so
this is just some basic clean-up.

If this feature was ever implemented again, the tests would need to be
rewritten anyway due to the changes in the way config is handled.

# User-Facing Changes

None

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`
- 
# After Submitting

N/A

tests/shell/environment/env.rs

@@ -1,10 +1,7 @@
-use super::support::Trusted;
-
 use nu_test_support::fs::Stub::FileWithContent;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, nu_repl_code, nu_with_std};
 use pretty_assertions::assert_eq;
-use serial_test::serial;
 
 #[test]
 fn env_shorthand() {
@@ -157,29 +154,6 @@ fn has_file_loc() {
     })
 }
 
-// FIXME: autoenv not currently implemented
-#[ignore]
-#[test]
-#[serial]
-fn passes_env_from_local_cfg_to_external_process() {
-    Playground::setup("autoenv_dir", |dirs, sandbox| {
-        sandbox.with_files(&[FileWithContent(
-            ".nu-env",
-            r#"[env]
-            FOO = "foo"
-            "#,
-        )]);
-
-        let actual = Trusted::in_path(&dirs, || {
-            nu!(cwd: dirs.test(), "
-                nu --testbin echo_env FOO
-            ")
-        });
-
-        assert_eq!(actual.out, "foo");
-    })
-}
-
 #[test]
 fn hides_env_in_block() {
     let inp = &[