Remove comment about -X and string exceptions. Error noted by

Justin D. Pettit <jpettit@raznick.com>.

Doc/lib/libexcs.tex

@@ -106,11 +106,10 @@ created with other than 2 or 3 arguments.  In this last case,
 \member{args} contains the verbatim constructor arguments as a tuple.
 \end{excdesc}
 
+
 \setindexsubitem{(built-in exception)}
 
 The following exceptions are the exceptions that are actually raised.
-They are class objects, except when the \programopt{-X} option is used
-to revert back to string-based standard exceptions.
 
 \begin{excdesc}{AssertionError}
 \stindex{assert}