Remove experimental Kafka metrics

[pyohannes]

Fixes #269

Changes

Based on a discussion in the semantic conventions SIG, this PR removes experimental Kafka metrics from the semantic conventions.

Reasons:

• The metrics in question were JMX specific, a different set of metrics is provided by other clients like e. g. Samara.
• Kafka folks currently have no intent maintaining a definition of Kafka broker metrics as part of OpenTelemetry semantic conventions.
• For very specific metrics, we'd like to encourage maintaining a definition of those metrics as part of the package that produces those metrics. For Kafka broker metrics, such a definition already seems to be available as part of the kafkametricsreceiver collector component.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.