Remove suggested reviewers secret and config

What does this MR do?

This MR removes suggested reviewers secret and config.

The Suggested Reviewers feature has been removed from GitLab Rails.

Related issues

https://gitlab.com/gitlab-org/gitlab/-/issues/553080+

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened - gitlab-org/omnibus-gitlab!8909

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.