refactor: delete non-prod code

So far the retainer backend type is always `built_in_database`.
The slightly over-engineered pre-implementation to support
another backend is likely not going to fly as the EMQX
resource frame work is mostly for auth and data integration.
i.e. not generic enough for retained messages.

apps/emqx_retainer/src/emqx_retainer.erl

@@ -425,17 +425,7 @@ get_backend_module() ->
 
 create_resource(Context, #{type := built_in_database} = Cfg) ->
     emqx_retainer_mnesia:create_resource(Cfg),
-    Context;
-create_resource(Context, #{type := DB} = Config) ->
-    ResourceID = erlang:iolist_to_binary([io_lib:format("~ts_~ts", [?APP, DB])]),
-    _ = emqx_resource:create(
-        ResourceID,
-        <<"emqx_retainer">>,
-        list_to_existing_atom(io_lib:format("~ts_~ts", [emqx_connector, DB])),
-        Config,
-        #{}
-    ),
-    Context#{resource_id => ResourceID}.
+    Context.
 
 -spec close_resource(context()) -> ok | {error, term()}.
 close_resource(#{resource_id := ResourceId}) ->