[Deprecation] Remove old _Backend enum

[MatthewBonanni]

Purpose

The old _Backend enum, now replaced by AttentionBackendEnum, was scheduled for removal in v0.13.0. This PR removes it.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[chatgpt-codex-connector]

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[mergify]

Documentation preview: https://vllm--30416.org.readthedocs.build/en/30416/

[gemini-code-assist]

Code Review

The pull request successfully removes the deprecated _Backend enum and its associated documentation entry, aligning with the planned deprecation schedule. This is a clean-up PR that improves code maintainability by removing old, unused code.

[MatthewBonanni]

Closing in favor of #30396