Remove severity and priority sections

Why is this change being made?

The current handbook page for the Create:Code Review backend team has grown to a size that makes it hard to use. I plan to start to break this into smaller pages that are hopefully more direct.

This MR removes the current sections for severity and priority under performance monitoring. The rules for these are defined on the quality/issue-triage part of the handbook, and that should be the SSOT.

Relates to: https://gitlab.com/gitlab-com/create-stage/code-review-be/-/issues/132

Review App

https://mnohr-cr-priority-severity.about.gitlab-review.app/handbook/engineering/development/dev/create/code-review/backend/

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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