Would be great to have the possibility to remove the "Select All" button from a Multiselect Choice (Optionset). Right now, it does not appear in field's configuration and if you want to remove it through Javascript is not possible (in JS is shown as being an option with null value). I think is good to give developers this possibility for scenarios when let's say after implementing some specific business logic, the end-user has to see only one option left in the field, in that case it does not make sense to still have the "Select All" checkbox visible.