Draft: Migrates buttons in dropdown helper to Pajamas view buttons

Note: Marking this as draft as it's blocked by the rollout of visible_label_selection_on_metadata

What does this MR do and why?

This MR updates the buttons in dropdowns_helper.rb to use Pajamas compliant buttons. It also removes some old, redundant code that wasn't being used.

Ideally, all dropdowns should stop using this helper and use a pajamas compliant dropdown instead, but this at least gets us a little closer.

Rabbit-hole on adding close and back buttons to dropdown headers

I removed the ability to add extra buttons to dropdown headers in the haml helper. Here's a video explainer on why that should be absolutely fine:

header_rabbit-hole.mov

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to your todo list in the GDK. All the dropdowns at the top of the page use this helper.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.