Removes unused Secret object

What does this MR do?

• The secret gitlab-hostkeys-credential-v1 was replaced with gitlab-hostkeys-v1
• All references are confirmed to have been removed, this is safe to delete across all clusters/environments

Closes: gitlab-com/gl-infra/delivery#2018 (closed)

Author Check-list

Please read the Contributing document and once you do, complete the following:

• Assign to the correct reviewer per the contributing document
• Apply the correct metadata per the contributing document
• Link to related MRs for applying the changes on other environments
• Link to related Chef changes
• If necessary link to a Criticality 4 Change Request issue

Reviewer Check-list

• Reviewed the diff jobs to confirm changes are as expected
• No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation