Remove opening/at-exit closing of devnull.

This was likely due to an issue on Python 2 seeing the date,
and does not seem useful anymore. Let's try to remove it.

Should close  #13896

IPython/utils/io.py

@@ -18,11 +18,6 b' from warnings import warn'
 from IPython.utils.decorators import undoc
 from .capture import CapturedIO, capture_output
 
-# setup stdin/stdout/stderr to sys.stdin/sys.stdout/sys.stderr
-devnull = open(os.devnull, "w", encoding="utf-8")
-atexit.register(devnull.close)
-
-
 class Tee(object):
     """A class to duplicate an output stream to stdout/err.