fixes #4714 - Removing traces of unused cdn_import_success code

This db column is left over legacy code that was being used from the days
when importing manifest also created repositories in pulp and candlepin.

After we moved to the better way of importing reposets this code became
dead code. Nothing was referring to this, so removing this.
Merge pull request #3838 from parthaa/product-cleanup

Removing traces of unused cdn_import_success code