deletion: fix issue in processing delete requests with tsdb index

[sandeepsukhani]

What this PR does / why we need it:
When a delete request creates a new chunk from an existing chunk, we index it to the index store.
In TSDB, we drop __name__ label, which was being added to chunkrefs for historical reasons.
However, this was not done while indexing chunks from the deletion code, causing deletion to fail with the error chunk insertion is only allowed on existing streams.

This PR handles the issue by dropping the __name__ label and adding relevant tests.

Special notes for your reviewer:
I will add an integration test for deletion with TSDB once we have PR #7754 merged since it does some groundwork to run integration tests with both boltdb-shipper and tsdb.

Checklist

• Tests updated
• CHANGELOG.md updated

[MichelHollands]

LGTM