Remove HTTP flushes for returning handlers (#13528)

When handlers return they are automatically flushed. Manual flushing can force responsewriters to use suboptimal paths and generally just wastes CPU.

cmd/admin-handlers-users.go

@@ -1307,9 +1307,6 @@ func (a adminAPIHandlers) ListBucketPolicies(w http.ResponseWriter, r *http.Requ
 		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, err), r.URL)
 		return
 	}
-
-	w.(http.Flusher).Flush()
-
 }
 
 // ListCannedPolicies - GET /minio/admin/v3/list-canned-policies
@@ -1342,8 +1339,6 @@ func (a adminAPIHandlers) ListCannedPolicies(w http.ResponseWriter, r *http.Requ
 		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, err), r.URL)
 		return
 	}
-
-	w.(http.Flusher).Flush()
 }
 
 // RemoveCannedPolicy - DELETE /minio/admin/v3/remove-canned-policy?name=<policy_name>