Don't log 'Exit code: 0'

A regular gate run shows this was logged ~2500 times.
It doesn't provide any useful information since the
command executed successfully and we can see the
issued command in the debug statement before it.

Partial-Bug: #1707307
Change-Id: I431d6a59ce36deb9a5fb9deef39655d28d71f24d

neutron/agent/linux/utils.py

@@ -149,8 +149,6 @@ def execute(cmd, process_input=None, addl_env=None,
                 LOG.error(msg)
             if check_exit_code:
                 raise ProcessExecutionError(msg, returncode=returncode)
-        else:
-            LOG.debug("Exit code: %d", returncode)
 
     finally:
         # NOTE(termie): this appears to be necessary to let the subprocess