[Chore] Remove more V0 dead code from sequence.py

[DarkLight1337]

Purpose

This code isn't used anymore so we can easily remove them.

We can move IntermediateTensors to a more suitable location in another PR.

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.

[gemini-code-assist]

Code Review

This pull request focuses on removing dead code, specifically the RequestMetrics dataclass and the constants VLLM_TOKEN_ID_ARRAY_TYPE and VLLM_INVALID_TOKEN_ID from vllm/sequence.py. Correspondingly, the usage of RequestMetrics has been removed from vllm/outputs.py. My review confirms that this code is indeed unused across the provided files. The changes are correct and contribute to better code maintainability by reducing clutter. The pull request is well-scoped and looks good to merge.