chore(mito): nit remove extra hashset in gc workers

[lyang24]

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What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed:

• Summarize your change (mandatory)
  i felt like the iter and collect into another hashset is not necessary as we can use the original in_used set
• How does this PR work? Need a brief introduction for the changed logic (optional)
• Describe clearly one logical change and avoid lazy messages (optional)
• Describe any limitations of the current code (optional)
• Describe if this PR will break API or data compatibility (optional)

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[waynexia]

💯 Thank you @lyang24