Doc: io: Remove "In-memory streams" section (GH-24927)

(cherry picked from commit b045cda)

Co-authored-by: Inada Naoki <[email protected]>

Author: miss-islington

Doc/library/io.rst

@@ -155,16 +155,6 @@ High-level Module Interface
    when an unsupported operation is called on a stream.
 
 
-In-memory streams
-^^^^^^^^^^^^^^^^^
-
-It is also possible to use a :class:`str` or :term:`bytes-like object` as a
-file for both reading and writing.  For strings :class:`StringIO` can be used
-like a file opened in text mode.  :class:`BytesIO` can be used like a file
-opened in binary mode.  Both provide full read-write capabilities with random
-access.
-
-
 .. seealso::
 
    :mod:`sys`