Remove `ignore_column` statement from vulnerabilities model

This MR remove the `ignore_column` as step 3 of the [3-M drop column
process][0]

All of the columns mentioned in the `ignore_column` removed in this
commit have been dropped in 16.8 as part of [task 431989][1]

[0]:https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#removing-the-ignore-rule-release-m2
[1]:https://gitlab.com/gitlab-org/gitlab/-/issues/431989

Resolves: https://gitlab.com/gitlab-org/gitlab/-/issues/268154