Removing unused DropdownButton

[Mephistic]

Summary

This component is entirely unused, so let's just delete it instead of bothering to update it to use Bootstrap/styled components.

Issue: #1457

Checklist

• [na] On the frontend, I've made my strings translate-able.
• [na] If I've added shared components, I've added a storybook story.
• [na] I've made pages responsive and look good on mobile.

Screenshots

N/A (No visible changes)

Known issues

The issue called out dropdown styles in the globals.scss file, but as far as I can tell, those only apply to a different dropdown (that does actually see use).

Steps to test/reproduce

N/A (Nothing uses this component)

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