Removed imported symbols that are not accessed or re-exported (#4387)

Co-authored-by: Eric Traut <erictr@microsoft.com>

stdlib/3/email/message.pyi

@@ -3,7 +3,6 @@
 from email.charset import Charset
 from email.contentmanager import ContentManager
 from email.errors import MessageDefect
-from email.header import Header
 from email.policy import Policy
 from typing import Any, Generator, Iterator, List, Optional, Sequence, Tuple, TypeVar, Union