fix: unset discount amount based on coupon code (backport #38250)

[mergify]

This is an automatic backport of pull request #38250 done by Mergify.
Cherry-pick of 6518582 has failed:

On branch mergify/bp/version-14-hotfix/pr-38250
Your branch is up to date with 'origin/version-14-hotfix'.

You are currently cherry-picking commit 6518582ed3.
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	deleted by us:   erpnext/public/js/utils/sales_common.js

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To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[frappe-pr-bot]

🎉 This PR is included in version 14.53.0 🎉

The release is available on GitHub release

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