chore: restore apps/emqx_retainer/src/emqx_retainer_sup.erl

this file, ce should be copied to ee, not the other way around

apps/emqx_retainer/src/emqx_retainer_sup.erl

@@ -34,9 +34,8 @@ init([Env]) ->
              type     => worker,
              modules  => [emqx_retainer]} || not is_managed_by_modules()]}}.
 
-%% This function is added to enterprise branch only.
-%% In enterprise edition, the retainer worker may start under modules supervisor
-%% so we should avoid starting it under the application root supervisor.
+-ifdef(EMQX_ENTERPRISE).
+
 is_managed_by_modules() ->
     try
         case supervisor:get_childspec(emqx_modules_sup, emqx_retainer) of
@@ -47,3 +46,11 @@ is_managed_by_modules() ->
         exit : {noproc, _} ->
             false
     end.
+
+-else.
+
+is_managed_by_modules() ->
+    %% always false for opensource edition
+    false.
+
+-endif.