In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21497, we
migrated all project import data into a separate table,
`project_import_data`.  In addition, we also added:

```
ignore_column :import_status, :import_jid, :import_error
```

In https://gitlab.com/gitlab-com/gl-infra/production/issues/908, we
observed some of these `import_error` columns consumed megabytes of
error backtraces and caused slow loading of projects whenever a `SELECT
* from projects` query loaded the row into memory.

Since we have long migrated away from these columns, we can now drop
these columns entirely.