paths: remove DEV_NULL

The platform-specific path to /dev/null is provided by the Python standard
library in os.devnull.

Replace all uses of paths.DEV_NULL with os.devnull and remove DEV_NULL.

https://fedorahosted.org/freeipa/ticket/6474

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipapython/ipautil.py

@@ -413,7 +413,7 @@ def run(args, stdin=None, raiseonerr=True, nolog=(), env=None,
     if stdin:
         p_in = subprocess.PIPE
     if skip_output:
-        p_out = p_err = open(paths.DEV_NULL, 'w')
+        p_out = p_err = open(os.devnull, 'w')
     elif redirect_output:
         p_out = sys.stdout
         p_err = sys.stderr