logger: remove NullHandler for the "slixmpp" handler

This does not seem to accomplish anything besides
hiding the "using the slow, pure python stringprep"
warning, unless you import logging and add another
handler before to the "slixmpp" logger *BEFORE*
importing slixmpp.

slixmpp/__init__.py

@@ -5,7 +5,6 @@
 # See the file LICENSE for copying permission.
 import logging
 from os import getenv
-logging.getLogger(__name__).addHandler(logging.NullHandler())
 
 # Use defusedxml if wanted
 # Since enabling it can have adverse consequences for the programs using