DEBUG(+) spam from gnome-session-manager@.service

gnome-session-manager@.service adds --debug to gnome-session, which triggers two undesirable behaviors:

1. It causes DEBUG(+) messages to be logged over syslog at WARNING level.
2. It causes duplicate DEBUG(+) messages to be logged over stderr at INFO level.
   • The stderr messages also contain a superfluous gnome-session-binary[PID]: prefix, probably from LOG_PERROR.