[etk] Remove deprecated hide-plugin-buttons-mobile fix

[fullofcaffeine]

Proposed Changes

Follow up to: #49329

Further context: p1661852852314429-slack-C7YPUHBB2

I don't think this fix is needed anymore. I did a quick test here, and it seems the display: none is not even being applied anymore, being superseded by other rules in core GB. Notice how the blue background is applied, but the element is still visible:

Peek.2022-09-01.13-00.mp4

Thanks @simison for reaching out about this.

Testing Instructions

• Checkout branch, sandbox a test site, and run yarn dev --sync
• Open a post or page in the block editor
• Shrink viewport down to mobile size and observe the plugin buttons disappear (they should still be available in the more menu)
• The menu layout should not break.

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-41731

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-41731&env=jetpack

[matticbot]

This PR modifies the release build for editing-toolkit

To test your changes on WordPress.com, run install-plugin.sh editing-toolkit remove/collapse-plugin-icons-on-mobile-deprecated-fix on your sandbox.

To deploy your changes after merging, see the documentation: PCYsg-mMA-p2

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.