Revert "Merge branch 'arkose-labs-captcha-header-guard' into 'master'"

This reverts merge request !83903 (merged)

The introduction of a new CAPTCHA challenge broke QA tests. In !83903 (merged), we tried to implement a way for QA tests to bypass the challenge based on an HTTP header. Unfortunately, that approach did not work as the header was not set when we expected it to be. To keep the code as clean as possible, we are now reverting that change until we can come up with a better solution. The CAPTCHA challenge remains hidden behind a disabled feature flag.