remove two unused variables

[zhjwpku]

and polish two comments which are not precise.

[sre-ci-robot]

Welcome @zhjwpku! It looks like this is your first PR to zilliztech/knowhere 🎉

[mergify]

@zhjwpku 🔍 Important: PR Classification Needed!

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1. If you're fixing a bug, label it as kind/bug.
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[zhjwpku]

/assign @cqy123456

[cqy123456]

/lgtm

[chasingegg]

/lgtm

Thanks for the contribution!

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chasingegg, zhjwpku

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[chasingegg]

/kind improvement