[geometry] DOMPoint constructor dictionary parameter should be optional

WebIDL spec says:

> If the type of an argument is a dictionary type or a union type that has a
> dictionary type as one of its flattened member types, and this argument is
> either the final argument or is followed only by optional arguments, then
> the argument MUST be specified as optional.
>
So the DOMPoint(DOMPointInit) constructor parameter needs to be optional.
This means the no-arg DOMPoint constructor must be removed.

Rob
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