Remove BigQuery credentials

[ethax-ross]

Context

We now pass the dfe-analytics credentials as part of CI/terraform; we no longer need to specify them in the Rails credentials.

Changes proposed in this pull request

• Remove BigQuery credentials

As we are not passing these in from the CI/terraform we no longer need to provide them as part of the Rails credentials.

Removes the following keys from review, sandbox, staging and production:

BIGQUERY_DFE_ANALYTICS_DATASET
BIGQUERY_DFE_ANALYTICS_API_JSON_KEY

Guidance to review

To make testing easier this is also removing the credentials that were used by the legacy BigQuery integration, which is being removed as part of #5496.

Checked by manually deploying to:

• Review ✅
• Staging ✅
• Sandbox ✅
• Production ✅

Then running a query such as:

SELECT occurred_at, request_query, request_path
FROM `ecf-bq.ecf_events_review.events` 
WHERE TIMESTAMP_TRUNC(occurred_at, DAY) = TIMESTAMP("2025-01-30") 
ORDER BY occurred_at 
DESC LIMIT 5

And hitting a path /users/sign_in?test=ross, which we should then see in the results:

[github-actions]

Review app deployed to https://cpd-ecf-review-5497-web.test.teacherservices.cloud