settings: cleanup

Cleanups:

- `$moodlehashideif` is always `true` because this plugin already requires Moodle 3.7+.
  - Remove "false" branches and tidy.
  - Remove `environmentlib.php` because we no longer use `normalize_version()`.
- Use notification constants instead of static strings. Requires Moodle 3.1.
- Use `new lang_string()` when not immediately using the string. Requires Moodle 2.3.
- Use admin_setting_configcheckbox_with_lock for locked fields. Requires Moodle 2.0.
- Avoid creating unnecessary local variables.

Author: jrchamp