Remove comment from metaschema that some fields are not implemented

[badboy]

It seems those fields are actually in use.

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

[badboy]

These fields were proposed in docs.google.com/document/d/1oNi-eX_IPIZa8C0h5GZnWTM0WFivZ1hPMgMMX3c3HHs/edit#heading=h.36jrolbv6nko It hasn't been fully implemented, i.e. pipeline still parses URI to determine destination dataset so these descriptions are correct.

Hm, I see. but they are in use by some parts though.