Docs: remove mention of perf testing from release docs

[bph]

What?

Removes mention of performance testing from the plugin release documentation.
Partial fix of #50343

Why?

The performance comparison testing between the Gutenberg plugin and WP version has been found unreliable, has been removed from the workflow of the release.

How?

Edit the release.md file

Testing Instructions

Use the view file feature from the PR.

[github-actions]

Flaky tests detected in b2fc8d5.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4885739593
📝 Reported issues:

• [Flaky Test] should switch to block tab if we select a block, when Template is selected #48878 in specs/site-editor/settings-sidebar.test.js