Remove raise statements from function bodies (#3355)

While it may eventually be useful to mark the exceptions that can be
raised from a function or method, the semantics are currently undefined
and unclear.

stdlib/2/thread.pyi

@@ -21,10 +21,8 @@ class _localdummy(object): ...
 def start_new(function: Callable[..., Any], args: Any, kwargs: Any = ...) -> int: ...
 def start_new_thread(function: Callable[..., Any], args: Any, kwargs: Any = ...) -> int: ...
 def interrupt_main() -> None: ...
-def exit() -> None:
-    raise SystemExit()
-def exit_thread() -> Any:
-    raise SystemExit()
+def exit() -> None: ...
+def exit_thread() -> Any: ...
 def allocate_lock() -> LockType: ...
 def get_ident() -> int: ...
 def stack_size(size: int = ...) -> int: ...