Revert "Change creationDate in telemetry.crash to date-time format (#816)"

[BenWu]

Reverts #816

The jsonschema-transpiler converts format: date-time fields to bigquery timestamps which makes sense because it's used for submission_timestamp but I thought creation_date was already a timestamp. Reverting because I don't see an easy low risk fix

Checklist for reviewer:

• [ ] Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• [ ] If adding a new field, the field should have a description (see #576 for an example)
• [ ] If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• [ ] Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• [ ] Follow the change control procedure