Strange debug prints appear in the system log
That is what the Syslog module that comes with Drupal does. It is not enabled by default, but enabling it will cause what you are noticing.
The module doesn't have its own settings to disable logging the messages into the system logs; to stop that, just disable the module. Since you are not going to use it anymore, you can then uninstall it. (Don't delete its files, since they are copied every time you update Drupal.)