Undo migration to remove redundant channel column

[soapy1]

Fixes #1070

Description

undoes: https://github.com/conda-incubator/conda-store/pull/1010/files

unfixes: #994

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

[netlify]

✅ Deploy Preview for conda-store canceled.

Name	Link
🔨 Latest commit	4cc5510
🔍 Latest deploy log	https://app.netlify.com/sites/conda-store/deploys/679c101733c9c400084b306d

[peytondmurray]

I checked by hand that there were no (significant) differences between orm.py and test_orm.py between this branch and 3250928. Not sure why git revert wasn't happy, but this effectively does the right thing anyway.