Clean binding generated code for custom attribute setters and getters

[youenn fablet]

As pointed out by Ossy in bug 150438, the binding generator code for custom attribute getters and/or setters should be fixed:

> > Source/WebCore/bindings/scripts/test/JS/JSTestJSBuiltinConstructor.cpp:158
> > +EncodedJSValue jsTestJSBuiltinConstructorTestAttributeCustom(ExecState* state, JSObject* slotBase, EncodedJSValue thisValue, PropertyName)
> > +{
> > +    UNUSED_PARAM(state);
> > +    UNUSED_PARAM(slotBase);
> > +    UNUSED_PARAM(thisValue);
> > +    auto* castedThis = jsCast<JSTestJSBuiltinConstructor*>(slotBase);
> > +    return JSValue::encode(castedThis->testAttributeCustom(*state));
> > +}
> 
> state and slotBase parameters are used. thisValue is really unused, 
> but in this case omitting the parameter name would be better.

[youenn fablet]

After a quick look at the  binding generator code for attribute getters/setters, it contains many "if" statements.
It is not always easy to identify whether each variable is used or not, depending on the different generation options.
Hence the current code I guess.

[youenn fablet]

*** This bug has been marked as a duplicate of bug 158068 ***