Revert "Move all IAM storage functionality into iam store type (#13541)"

This reverts commit caadcc3ed8.

cmd/admin-handler-utils.go

@@ -103,12 +103,6 @@ func toAdminAPIErr(ctx context.Context, err error) APIError {
 				Description:    err.Error(),
 				HTTPStatusCode: http.StatusServiceUnavailable,
 			}
-		case errors.Is(err, errPolicyInUse):
-			apiErr = APIError{
-				Code:           "XMinioAdminPolicyInUse",
-				Description:    "The policy cannot be removed, as it is in use",
-				HTTPStatusCode: http.StatusBadRequest,
-			}
 		case errors.Is(err, kes.ErrKeyExists):
 			apiErr = APIError{
 				Code:           "XMinioKMSKeyExists",