rabbit_config: Deprecate schema_dir/0

It is unused in RabbitMQ or tier-1 plugins, and the previously returned
value made no sense since the switch to Cuttlefish as a library (as part
of #2180).

Author: dumbbell

src/rabbit_config.erl

@@ -1,11 +1,13 @@
 -module(rabbit_config).
 
 -export([
-         schema_dir/0,
          config_files/0,
          get_advanced_config/0
         ]).
 
+-export([schema_dir/0]).
+-deprecated([{schema_dir, 0, eventually}]).
+
 -export_type([config_location/0]).
 
 -type config_location() :: string().
@@ -17,14 +19,7 @@ get_confs() ->
     end.
 
 schema_dir() ->
-    case init:get_argument(conf_schema_dir) of
-        {ok, SchemaDir} -> SchemaDir;
-        _ ->
-            case code:priv_dir(rabbit) of
-                {error, bad_name} -> filename:join([".", "priv", "schema"]);
-                PrivDir           -> filename:join([PrivDir, "schema"])
-            end
-    end.
+    undefined.
 
 get_advanced_config() ->
     case get_prelaunch_config_state() of