Remove non-glean pioneer/rally studies and metadata

[whd]

https://mozilla-hub.atlassian.net/browse/DSRE-1173

Requires mozilla/mozilla-schema-generator#274 and mozilla/probe-scraper#781 since this will be getting rid of associated metadata schemas as well. There are a few rounds of other followup PRs (MSG again, bqetl) after this first round of PRs lands.

This PR also deletes the Pioneer v1 pioneer-study ping from the main data platform, which shouldn't cause any issues given https://github.com/mozilla/gcp-ingestion/blob/main/ingestion-beam/src/main/java/com/mozilla/telemetry/decoder/MessageScrubber.java#L125

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 Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #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

[akkomar]

👍