Revert "Merge branch '388156-read-vuln-custom-role' into 'master'"

What does this MR do and why?

This reverts merge request !114734 (merged) due to a performance issue (caught on canary, only for some groups and group owners).

The problem is caused by Member.elevated_guests method. The performance issue is new even though there was no change to this method or related queries. The reason is that before adding read_vulnerabilities to custom roles, we had only read_code permission for custom roles which is excluded from the elevating_permissions.

We need to put elevated_guests under a FF and long-term we need to fix the performance issue.

Db migration

In the original MR there was a migration that added a new column. Revert of this migration is not included as that didn't cause any issues and the migration will be finished on the production once this one gets there.

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