[config reload] Removed job-mode for sonic.target restart (#1820)

#### What I did

As discussed in this PR #1814 (comment), only the stop.job should have job-mode set to replace irreversibly.

Otherwise,  simultaneous config reloads in the quick succession, can lead to the behavior.

Although ,when the restart job (and all the other dependent jobs) are finished and is taken out of systemd's job queue, the next stop job will not be cancelled.

config/main.py

@@ -706,7 +706,7 @@ def _reset_failed_services():
 
 def _restart_services():
     click.echo("Restarting SONiC target ...")
-    clicommon.run_command("sudo systemctl restart sonic.target --job-mode replace-irreversibly")
+    clicommon.run_command("sudo systemctl restart sonic.target")
 
     try:
         subprocess.check_call("sudo monit status", shell=True, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)