removed obsolete get_sg;

--- a/src/Pure/context.ML	Mon Apr 30 13:32:58 2007 +0200
+++ b/src/Pure/context.ML	Mon Apr 30 13:33:00 2007 +0200
@@ -569,7 +569,6 @@
   val init: theory -> theory
   val print: theory -> unit
   val get: theory -> T
-  val get_sg: theory -> T    (*obsolete*)
   val put: T -> theory -> theory
   val map: (T -> T) -> theory -> theory
 end;