[heycam/webidl] ReadWriteAttribute production should not allow using "readonly" with inherit (#703)

Right now we have:
```
ReadWriteAttribute ::
    inherit ReadOnly AttributeRest
    AttributeRest
```

I think this should be:
```
ReadWriteAttribute ::
    inherit AttributeRest
    AttributeRest
```

because allowing "inherit readonly attribute type name;" in the grammar just to disallow it in prose is a little silly.

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