Remove references using GitLab to install Sentry

What does this MR do?

Removes final mentions of installing Sentry with the cluster management project template. Based on the fact that the documentation page for Sentry app had already been removed, it looks like we just forgot to remove these references (as well as gitlab-org/project-templates/cluster-management!65 (merged))

Related issues and Mrs

gitlab-org/project-templates/cluster-management!65 (merged)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
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  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.