Revert sum literal integer change (#13961)

This is allegedly causing large performance problems, see 13821

typeshed/8231 had zero hits on mypy_primer, so it's not the worst thing
to undo. Patching this in typeshed also feels weird, since there's a
more general soundness issue. If a typevar has a bound or constraint, we
might not want to solve it to a Literal.

If we can confirm the performance regression or fix the unsoundness
within mypy, I might pursue upstreaming this in typeshed.

(Reminder: add this to the sync_typeshed script once merged)

mypy/typeshed/stdlib/builtins.pyi

@@ -1662,7 +1662,7 @@ _SupportsSumNoDefaultT = TypeVar("_SupportsSumNoDefaultT", bound=_SupportsSumWit
 # without creating many false-positive errors (see #7578).
 # Instead, we special-case the most common examples of this: bool and literal integers.
 @overload
-def sum(iterable: Iterable[bool | _LiteralInteger], /, start: int = 0) -> int: ...
+def sum(iterable: Iterable[bool], /, start: int = 0) -> int: ...
 @overload
 def sum(iterable: Iterable[_SupportsSumNoDefaultT], /) -> _SupportsSumNoDefaultT | Literal[0]: ...
 @overload