cloudinit.log: Don't configure NullHandler (#4435)

NullHandler hasn't been required to prevent the logging warnings since
commit 5a27d4018 landed in Python, which was released in Python 3.2.

cloudinit/log.py

@@ -113,19 +113,14 @@ def setupLogging(cfg=None):
 
 
 def resetLogging():
-    """Remove all current handlers, unset log level and add a NullHandler.
-
-    (Adding the NullHandler avoids "No handlers could be found for logger XXX"
-    messages.)
-    """
+    """Remove all current handlers and unset log level."""
     log = logging.getLogger()
     handlers = list(log.handlers)
     for h in handlers:
         h.flush()
         h.close()
         log.removeHandler(h)
     log.setLevel(logging.NOTSET)
-    log.addHandler(logging.NullHandler())
 
 
 def setup_backup_logging():