Delete many redundant method redefinitions (#6877)

stdlib/email/message.pyi

@@ -17,7 +17,6 @@ class Message:
     preamble: str | None
     epilogue: str | None
     defects: list[MessageDefect]
-    def __str__(self) -> str: ...
     def is_multipart(self) -> bool: ...
     def set_unixfrom(self, unixfrom: str) -> None: ...
     def get_unixfrom(self) -> str | None: ...