Remove wrong TODO note

python/mypy#1178 is about variable-length tuples, while exc_info()
always returns a tuple with length 3. Ideally, exc_info() would
return Union[Tuple[Type[_E], _E, TracebackType], Tuple[None, None, None]],
but that is a different issue.

stdlib/2/sys.pyi

@@ -116,7 +116,6 @@ def __displayhook__(value: int) -> None: ...
 def __excepthook__(type_: type, value: BaseException, traceback: TracebackType) -> None: ...
 def exc_clear() -> None:
     raise DeprecationWarning()
-# TODO should be a union of tuple, see mypy#1178
 def exc_info() -> _OptExcInfo: ...
 
 # sys.exit() accepts an optional argument of anything printable