bpo-31450: Remove documentation mentioning that subprocess's child_traceback is…

bpo-31450: Remove documentation mentioning that subprocess's child_traceback is available with the parent process (GH-11422)

Doc/library/subprocess.rst

@@ -572,9 +572,7 @@ Exceptions
 ^^^^^^^^^^
 
 Exceptions raised in the child process, before the new program has started to
-execute, will be re-raised in the parent.  Additionally, the exception object
-will have one extra attribute called :attr:`child_traceback`, which is a string
-containing traceback information from the child's point of view.
+execute, will be re-raised in the parent.
 
 The most common exception raised is :exc:`OSError`.  This occurs, for example,
 when trying to execute a non-existent file.  Applications should prepare for