Remove unnecessary `_getCustomClass` method.
`customClass` will be in the configuration already, even if it is provided in the data attributes, and jQuery's [`addClass()`](https://api.jquery.com/addclass/) method will handle it, either `this.config.customClass` is a function or a class name (even if it is a blank string).
Co-authored-by: XhmikosR <xhmikosr@gmail.com>
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