This change deduplicates the ACL policy list generated from ACL
roles referenced within an ACL token on the client.

Previously the list could contain duplicates, which would cause
erronous permission denied errors when calling client related RPC/
HTTP API endpoints. This is because the client calls the ACL get
policies endpoint which subsequently ensures the caller has
permission to view the ACL policies. This check is performed by
comparing the requested list args with the policies referenced by
the caller ACL token. When a duplicate is present, this check
fails, as the check must ensure the slices match exactly.