[stable-5] Remove unneccessary imports (#5977)

* Remove unneccessary imports (#5940)

* Remove unneccessary imports.

* Keep unnecessary imports in module_utils - for now.

* Make older sanity tests shut up.

* Also make flake8 happier.

(cherry picked from commit 2b8ac3c629)

* Re-add os import.

plugins/callback/mail.py

@@ -78,7 +78,6 @@ import re
 import email.utils
 import smtplib
 
-from ansible.module_utils.six import string_types
 from ansible.module_utils.common.text.converters import to_bytes
 from ansible.parsing.ajson import AnsibleJSONEncoder
 from ansible.plugins.callback import CallbackBase