Remove un-used attributes from the Collection table

Remove:
- publish_url
- pending_url
- summary
- description
None of them were used on pkgdb1 and none of them are used in the current
scheme of pkgdb2.
Do not port removed field from pkgdb1 to pkgdb2

Since we remove publish_url, pending_url, summary and description, we can
no longer import them from pkgdb1 into pkgdb2.