[serial watchdog] remove serial watchdog service dependency to rc.local (sonic-net#1752)

* [serial watchdog] remove serial watchdog service dependency to rc.local

When restarting this service in rc.local, the dependency causes an error
in syslog. Removing the dependency to mute the error log entry.

* remove lines with empty inputs

Author: yxieca

files/image_config/serial-port-watchdog/serial-port-watchdog.service

@@ -1,7 +1,5 @@
 [Unit]
 Description=Monitor serial port processes, kill stuck ones
-Requires=
-After=rc.local.Service
 
 [Service]
 ExecStart=/usr/bin/serial-port-watchdog.py -t ttyS0