fix: Remove duplicated metric from Kafka producer

[benclive]

What this PR does / why we need it:
Removes the loki_kafka_client_buffered_produce_bytes metric from Loki's Kafka producer code.

• The metric tracks the current number of bytes buffered in the client. It stays fairly constant over time
• A metric of the same name is being produced by the Kafka client directly after the latest update.
• The new metric is calling exactly the same method as our custom one (client.BufferedProduceBytes() @ vendor/github.com/twmb/franz-go/plugin/kprom/kprom.go)
• The only difference is that the new metric is a Gauge and the old metric is a Summary. However, a quick search of deployment_tools shows we aren't using this metric anywhere in our dashboards so I don't think change is important.