Remove out-dated information from DEVELOPERS docs

[thingalon]

The old release instructions were still in DEVELOPERS.md, and no longer apply since moving to the Monorepo.

Changes proposed in this Pull Request:

• Remove out-dated release procedure from documentation.

Does this pull request change what data or activity we track or use?

no

Testing instructions:

n/a

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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• ✅ Add changelog entries to affected projects
  

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Boost plugin:

• Next scheduled release: September 7, 2021.
• Scheduled code freeze: August 30, 2021.