Breaking: Remove spamcheck subchart

What does this MR do?

Removal of Spamcheck subchart as planned, announced deprecation for %19.0 in https://docs.gitlab.com/update/deprecations/#spamcheck-support-in-the-linux-package-and-gitlab-helm-chart

Related issues

Closes GitLab chart: Remove spamcheck subchart (#6471 - closed)

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.

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.
  • Spec in Operator is to be updated with Skip Spamcheck on Chart 10 and up (gitlab-org/cloud-native/gitlab-operator!1466 - merged)