varlink: fix dead code

'e' is already checked in the previous branch, no need to check it again,
it cannot be NULL.

CID#1534465

Follow-up for 9bca989183

src/shared/varlink.c

@@ -2397,7 +2397,7 @@ int varlink_collect_full(
                                 if (ret_parameters)
                                         *ret_parameters = p;
                                 if (ret_error_id)
-                                        *ret_error_id = e ? json_variant_string(e) : NULL;
+                                        *ret_error_id = json_variant_string(e);
                                 if (ret_flags)
                                         *ret_flags = v->current_reply_flags;