Revert "fix legacy plugin output"

[stakira]

Reverts #1577

[Copilot]

Pull Request Overview

This PR reverts a previous fix related to legacy plugin output handling. The revert undoes changes that were made to improve legacy plugin output formatting, returning the code to its previous state.

• Removes automatic envelope data writing for legacy plugin notes
• Reverts note numbering system back to using count-based indexing instead of sequence position
• Updates test expectations to match the reverted behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
OpenUtau.Test/Classic/PluginRunnerTest.cs	Updates test expectations to remove envelope data and revert note numbering
OpenUtau.Core/Classic/UstNote.cs	Removes automatic envelope writing for plugin output
OpenUtau.Core/Classic/Ust.cs	Reverts note numbering from sequence-based to count-based indexing

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