Remove mention of global prefetch

[johanrhodin]

Proposed Changes

RabbitMQ 4.0 removes Global prefetch (https://www.rabbitmq.com/blog/2021/08/21/4.0-deprecation-announcements#removal-of-global-qos). This is a step to remove mentions of global prefetch in the in-product docs.

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• [] All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

https://www.rabbitmq.com/docs/consumer-prefetch would need more in-depth changes.

[michaelklishin]

This removes the mention in one place but leaves

• A channel metric
• A Prometheus gauge
• At least one other management UI
• A rabbitmqctl list_channels info item (field)