Remove no-longer-needed convert_env_values calls (#14681)

# Description

Takes advantage of #14591 to remove the now-necessary calls to
`convert_env_values()` that I added in #14249. The function is now just
called once to convert `PATH`.

Also removed the Windows-build-time checks for `ensure_path`, since
previous case-insensitivity fixes make this unnecessary as well.

# User-Facing Changes

None - #14591 now handles conversion 'on-demand'.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

src/config_files.rs

@@ -51,18 +51,6 @@ pub(crate) fn read_config_file(
             use_color
         );
     } else {
-        let start_time = std::time::Instant::now();
-        let config = engine_state.get_config();
-        let use_color = config.use_ansi_coloring.get(engine_state);
-        if let Err(e) = convert_env_values(engine_state, stack) {
-            report_shell_error(engine_state, &e);
-        }
-        perf!(
-            "translate env vars before default_config.nu",
-            start_time,
-            use_color
-        );
-
         eval_default_config(engine_state, stack, get_default_config(), is_env_config);
     };