Revise allowed roles for label element

[scottaohara]

the rules around what roles are allowed on label should better refelct when adjusting the role will cause an accessibility issue.

there are really 3 differnet use cases:

1. label is associated with a form element via its for attribute
2. label is associated with a form element by being its ancestor
3. label is not associated with a form element

For 1 & 2, there can be accName breakages if the role is changed - and for 2 specifically there can be potential invalid nesting of role issues we would want to avoid

For 3.... someone decided to use label instead of span. For most cases, I'd submit there's not a whole lot that can go wrong here if someone were to modify the role so the element could be exposed as / behave as something else.