fix(ras-acc): remove unneeded aria-label from checkout-button

[laurelfulford]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR removes an unnecessary aria-label attribute from the Checkout Button block -- since the button displays a text label, and the aria-label and visible button label value are the same, they're kind of redundant. (More info about `aria-label here).

It also has the nice side effect of fixing an issue where Checkout Button Blocks using inline highlight styles ended up kind of garbled.

See 1207817176293825-as-1208200566409339.

How to test the changes in this Pull Request:

1. Starting on trunk, add some Checkout Button blocks to a page. Use the 'Highlight' style on the block toolbar to change the text colour of one of the blocks.
2. Switch to the epic/ras-acc branch, and note how the button looks -- the <mark> tag and attributes added by the Highlight style end up inside of the aria-label attribute, mangling the HTML.

3. Switch to this branch.
4. Confirm that the output of the button now looks correct:

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

[laurelfulford]

Thanks @chickenn00dle! 🙌