Revert "Fix LDAP sync member removal on server errors"

Purpose of revert

Reverts !224135 (merged). Per customer feedback, the behavioral change needs to be reimplemented behind a feature flag so self-managed admins can control when to enable it.

Checklist

• Create an issue to reinstate the merge request and assign it to the author of the reverted merge request.
  • Follow-up MR: !226913 (merged)
• If the revert is to resolve a broken 'master' incident, please read through the Responsibilities of the Broken master resolution DRI. N/A
• If the revert involves a database migration, please read through Deleting existing migrations. N/A

Milestone info

• I am reverting something in the current milestone. No changelog is needed, and I've added a ~"regression:*" label.

Related issues and merge requests

• Reverts: !224135 (merged)
• Follow-up: !226913 (merged)
• Issue: #6054