Remove lmp-testplan from the docs

[mwasilew]

Internal LmP test plan is now obsolete and should be removed from the docs.

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.
You may delete items that are not relevant to your contribution.
Readiness and adding reviewers as appropriate is required.

All PRs should be reviewed by a technical writer/documentation team and a peer.
If effecting customers—which is a majority of content changes—a member of Customer Success must also review.

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• Merge after date or event
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Checklist

• Run spelling and grammar check, preferably with linter.
• Step through instructions (or ask someone to do so).
• Review for wordiness
• Match tone and style of page/section.
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Comments

Any thing else that a maintainer/reviewer should know.
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[kprosise]

LGTM. After this is merged, I can follow up with adding a redirect.