Can't remove link to last-accessed public body on contact page

[mdeuk]

When visiting /help/contact after visiting a public body page (perhaps coincidentally), you cannot remove the "Link to authority" without first acknowledging the checkbox and completing both form fields.
This is a bit counterintuitive, and makes for a slightly confusing user experience.

Steps to reproduce

1. Visit any public body page, e.g. BBC or Heriot-Watt University
2. Scroll to the bottom of the page and click on "Contact Us"
3. Choose the "I have another issue…" option to bring the contact us form up
4. Note that the "Include link to authority" is automatically added.
5. Try clicking on "remove" - note that a prompt appears above the checkbox
6. Tick the checkbox and then try again - note a similar issue with the other two fields.

Example:

This is always reproducible.

Expected result

The user should be able to remove the link to the authority without having to complete the form beforehand.

This is similar to #443, however it manifests in a slightly different form.