fix: cancelation in kafkv2/consumer.go

[grobinson-grafana]

What this PR does / why we need it:

This commit fixes how we do cancelation in kafkav2/consumer.go. There was a select which wasn't needed as b.Ongoing() checks for ctx.Done(), and then we also returned a non-nil error if PollRecords was aborted, which would have been reported as a service failure at shutdown.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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