Don't remove traceback on rethrow

[slozier]

• Fixes traceback incorrect with contextlib.contextmanager #736
• Remove MakeRethrowExceptionWorker which is no longer necessary after Save/restore exc_info in context manager #735

[slozier]

Resolves the issue, but breaks the traceback of:

def g():
    f()

def f():
    try:
        raise Exception
    except:
        raise

g()