Feat: Improve FlexSlider initialization and element selection in JavaScript

- Updated the initialization of FlexSlider by using a regular function instead of an arrow function in the `flexslider.load.js` and `flexslider.load.min.js` files.
- Updated the element selection in the `flexslider.load.js` and `flexslider.load.min.js` files to use `$(this)` instead of `$(once(...))` and `$(_this)` respectively.

Changes:
- assets/js/flexslider.load.js: Improved FlexSlider initialization and element selection.
- dist/js/flexslider.load.js: Improved FlexSlider initialization and element selection.
- dist/js/flexslider.load.min.js: Improved FlexSlider initialization and element selection.