removed old comment;

--- a/src/Pure/Syntax/type_ext.ML	Wed Oct 12 16:34:00 1994 +0100
+++ b/src/Pure/Syntax/type_ext.ML	Wed Oct 12 16:34:52 1994 +0100
@@ -3,9 +3,6 @@
     Author:     Tobias Nipkow and Markus Wenzel, TU Muenchen
 
 The concrete syntax of types (used to bootstrap Pure).
-
-TODO:
-  term_of_typ: prune sorts
 *)
 
 signature TYPE_EXT0 =