Remove `icon_utils.js` and methods

What does this MR do and why?

Now that we have migrated the toolbox SVGs to the gitlab-ui and removed the getToolboxOptions method, we no longer need these additional utilities to load SVGs into the DOM.

This MR follows on from:

• feat(Charts): use `options.toolbox.show` to sho... (gitlab-ui!4352 - merged)
• Replace `getToolboxOptions` usages with `option... (!159188 - merged)

Closes #469191 (closed) #469190 (closed) #469196 (closed)

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