Replace jQuery cleanup functions with vanilla JS

[marcustyphoon]

Here's a totally random idea I had: rather than using jQuery's ergonomic "do XYZ to every element matched by this selector" functions in clean(), we could import them from a utility file.

This is mostly because I occasionally mistype $(`.${className}`).removeClass(className), or have to pause to dash-case a data attribute, and because VS Code automatically imports XKit Rewritten's utility files when it autocompletes.

User-facing changes

• n/a

Technical explanation

n/a

Issues this closes

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[marcustyphoon]

(currently untested ~~and unimplemented; will do properly if you are interested~~)