Remove custom id from further condition textareas so that labels match

[stevehook]

Context

The custom id causes a mismatch between the label (that we can't specify a custom
id for) and the textarea itself.

Changes proposed in this pull request

No visible change, removing the id generates the following kind of HTML:

<div class="govuk-form-group">
  <label for="make-an-offer-further-conditions0-field" class="govuk-label govuk-label--s">First condition</label>
  <textarea id="make-an-offer-further-conditions0-field" class="govuk-textarea" rows="3" name="make_an_offer[further_conditions0]"></textarea>
</div>

Guidance to review

The id value only seemed to be referred to in a system spec and we can use a name there instead. Any other reason why we needed the id?

Link to Trello card

https://trello.com/c/BWbCBEVj/693-dac-page-15-add-form-labels-to-provider-offer-page-conditions

Things to check

• This code doesn't rely on migrations in the same Pull Request
• API release notes have been updated if necessary
• New environment variables have been added to the Azure config