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/mod.rs

@@ -1,22 +1 @@
 mod env;
-
-// FIXME: nu_env tests depend on autoenv which hasn't been ported yet
-// mod nu_env;
-
-pub mod support {
-    use nu_test_support::{nu, playground::*, Outcome};
-
-    pub struct Trusted;
-
-    impl Trusted {
-        pub fn in_path(dirs: &Dirs, block: impl FnOnce() -> Outcome) -> Outcome {
-            let for_env_manifest = dirs.test().to_string_lossy();
-
-            nu!(cwd: dirs.root(), "autoenv trust \"{}\"", for_env_manifest);
-            let out = block();
-            nu!(cwd: dirs.root(), "autoenv untrust \"{}\"", for_env_manifest);
-
-            out
-        }
-    }
-}