Passing on validators to select subfields seems wrong

This was added in https://github.com/wtforms/wtforms/pull/615 but it breaks custom select field subclasses that use e.g. a list of checkboxes as they now all get the required attribute.

Here's a failing test case (since both inputs get the required flag):

def test_required_flag_custom():
    class CustomWidget:
        def __call__(self, field, **kwargs):
            html = []
            for subfield in field:
                html.append(f'{subfield.label}: {subfield()}')
            return ''.join(html)

    class CustomSelectMultipleField(SelectMultipleField):
        widget = CustomWidget()
        option_widget = CheckboxInput()

    F = make_form(
        c=CustomSelectMultipleField(
            choices=[("a", "hello"), ("b", "bye")],
            validators=[validators.DataRequired()],
        )
    )
    form = F(DummyPostData(c=["a"]))
    assert form.c() == (
        '<label for="c-0">hello</label>: <input checked id="c-0" name="c" type="checkbox" value="a">'
        '<label for="c-1">bye</label>: <input id="c-1" name="c" type="checkbox" value="b">'
    )

I have the feeling that passing validators to subfields isn't the correct thing to do - only <input type="radio"> handles the required flag correctly, but of course for multi-select you'll never have radio buttons...

Start with SelectMultipleField and the supplied test_required_flag_custom reproducer, then trace how validators reach its subfields and how the custom checkbox widget renders them. Done means the regression test passes without adding required to every checkbox while multi-select validation still works.