refactor: node.config_files should be hidden as it's not configurable

this config schema exists only for app-env mapping.
users can not change it because the bootstraping config
has to be etc/emqx.conf

apps/emqx_conf/i18n/emqx_conf_schema.conf

@@ -456,19 +456,6 @@ the old dir should be deleted first.<br/>
     }
   }
 
-  node_config_files {
-    desc {
-      en: """List of configuration files that are read during startup. The order is
-significant: later configuration files override the previous ones.
-          """
-      zh: """启动时读取的配置文件列表。后面的配置文件项覆盖前面的文件。"""
-    }
-    label {
-      en: "Config Files"
-      zh: "配置文件"
-    }
-  }
-
   node_global_gc_interval {
     desc {
       en: """Periodic garbage collection interval. Set to <code>disabled</code> to have it disabled."""

apps/emqx_conf/src/emqx_conf_schema.erl

@@ -466,9 +466,9 @@ fields("node") ->
                 hoconsc:array(string()),
                 #{
                     mapping => "emqx.config_files",
-                    default => undefined,
-                    'readOnly' => true,
-                    desc => ?DESC(node_config_files)
+                    hidden => true,
+                    required => false,
+                    'readOnly' => true
                 }
             )},
         {"global_gc_interval",
@@ -1037,17 +1037,13 @@ metrics_enabled(disabled) -> [].
 tr_default_config_driver(Conf) ->
     conf_get("rpc.driver", Conf).
 
-tr_config_files(Conf) ->
-    case conf_get("emqx.config_files", Conf) of
-        [_ | _] = Files ->
-            Files;
-        _ ->
-            case os:getenv("EMQX_ETC_DIR") of
-                false ->
-                    [filename:join([code:lib_dir(emqx), "etc", "emqx.conf"])];
-                Dir ->
-                    [filename:join([Dir, "emqx.conf"])]
-            end
+tr_config_files(_Conf) ->
+    case os:getenv("EMQX_ETC_DIR") of
+        false ->
+            %% testing, or running emqx app as deps
+            [filename:join([code:lib_dir(emqx), "etc", "emqx.conf"])];
+        Dir ->
+            [filename:join([Dir, "emqx.conf"])]
     end.
 
 tr_cluster_override_conf_file(Conf) ->