Remove obsolete mention of IRIX native threads -- we don't use them anymore.
diff --git a/Doc/library/_thread.rst b/Doc/library/_thread.rst
index 80fad68..7f25dd6 100644
--- a/Doc/library/_thread.rst
+++ b/Doc/library/_thread.rst
@@ -176,12 +176,10 @@
* It is not possible to interrupt the :meth:`acquire` method on a lock --- the
:exc:`KeyboardInterrupt` exception will happen after the lock has been acquired.
- .. index:: pair: threads; IRIX
-
* When the main thread exits, it is system defined whether the other threads
- survive. On SGI IRIX using the native thread implementation, they survive. On
- most other systems, they are killed without executing :keyword:`try` ...
- :keyword:`finally` clauses or executing object destructors.
+ survive. On most systems, they are killed without executing
+ :keyword:`try` ... :keyword:`finally` clauses or executing object
+ destructors.
* When the main thread exits, it does not do any of its usual cleanup (except
that :keyword:`try` ... :keyword:`finally` clauses are honored), and the