Remove unused constant: `Vulnerability::TooManyDaysError`

1. This was added in commit [5825c12f][0]
2. It was also used in [7ff151eb][1]
3. The resolver from 2 removed in [89009fc7][2]
4. The usage from 1 was removed in [e9567195][3]

[0]:https://gitlab.com/gitlab-org/gitlab/-/commit/5825c12f9a99dc5d9044740d688d2b36d6f1cfb8
[1]:https://gitlab.com/gitlab-org/gitlab/-/commit/7ff151eb0522a06e1d3843a0915f32c2304adc7b
[2]:https://gitlab.com/gitlab-org/gitlab/-/commit/89009fc7e7c4830dbebec7ccd911fb4fcee17722
[3]:https://gitlab.com/gitlab-org/gitlab/-/commit/e95671953d747eed9bda479bd3c1b49d6a3149ae

resolves: https://gitlab.com/gitlab-org/gitlab/-/issues/432922