[Backport 2.x] Remove identity-related feature flagged code from the RestController (#15430)#16004

reta merged 2 commits intoopensearch-project:2.xopensearch-project/OpenSearch:2.xfrom

cwperks:backport-15430-to-2.xcwperks/OpenSearch:backport-15430-to-2.xCopy head branch name to clipboard