Resolve "Only prompt user once when navigating away from file editor"

What does this MR do?

Removes beforeunload event listener when clicking the cancel button.

What are the relevant issue numbers?

Closes #55344 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the style guides
• Security reports checked/validated by reviewer