Revert "Add shortened customer bank statement"

[cesarcosta99]

Changes proposed in this Pull Request

We've stumbled upon questions on #3643 that may require changes on how we save the short statement descriptor in the settings. In order to avoid messing up with the settings structure, and generate compatibility issues, this PR is reverting the changes that added the shortened statement descriptor field.

Testing instructions

In case you'd like to double-check:

1. Go to the WCPay settings.
2. There is no Shortened customer bank statement field below the Full bank statement.
3. Open the dev tools and reload the page.
4. In the GET /wp-json/wc/v3/payments/settings request, there is no is_short_statement_descriptor_enabled and short_statement_descriptor in the response.
5. Save the settings.
6. In the POST /wp-json/wc/v3/payments/settings request, there is no is_short_statement_descriptor_enabled and short_statement_descriptor in the payload.

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• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc : Add link here / 'QA Testing Not Applicable'

[reykjalin]

LGTM 🎉