removed obfuscating PStrStrTab

--- a/src/Pure/General/table.ML	Fri Oct 28 08:40:55 2005 +0200
+++ b/src/Pure/General/table.ML	Fri Oct 28 09:35:04 2005 +0200
@@ -389,7 +389,3 @@
 
 structure Inttab = TableFun(type key = int val ord = int_ord);
 structure Symtab = TableFun(type key = string val ord = fast_string_ord);
-structure PStrStrTab = TableFun(
-  type key = string * string
-  val ord = prod_ord fast_string_ord fast_string_ord
-);