do not export Attrib.register_config, to make it harder to use low-level Config.declare after the bootstrap phase;

--- a/src/Pure/Isar/attrib.ML	Sun Mar 28 15:13:19 2010 +0200
+++ b/src/Pure/Isar/attrib.ML	Sun Mar 28 15:38:07 2010 +0200
@@ -36,7 +36,6 @@
   val multi_thm: thm list context_parser
   val print_configs: Proof.context -> unit
   val internal: (morphism -> attribute) -> src
-  val register_config: Config.value Config.T -> theory -> theory
   val config_bool: bstring -> bool -> bool Config.T * (theory -> theory)
   val config_int: bstring -> int -> int Config.T * (theory -> theory)
   val config_string: bstring -> string -> string Config.T * (theory -> theory)