$usedOnly to $onlyUsed

I found different "labels" for the same thing...

at DCA `onlyused` for checkbox "Assigned values only" but we have "transition" to value `$usedOnly`

e.g.
https://github.com/MetaModels/core/blob/master/src/MetaModels/Filter/Setting/SimpleLookup.php#L103
to
https://github.com/MetaModels/core/blob/master/src/MetaModels/Attribute/BaseSimple.php#L87