data: Don't start gnome-session-binary with debugging on systemd

For debugging reason, --debug was passed to gnome-session-binary in the
systemd service file. This is not necessariy and just causes a lot of
debug spew to be logged on systems.

Simply remove it again.

Fixes: #31

data/gnome-session-manager@.service.in

@@ -15,5 +15,5 @@ Before=gnome-session-manager.target
 
 [Service]
 Type=notify
-ExecStart=@libexecdir@/gnome-session-binary --debug --systemd-service --session=%i
+ExecStart=@libexecdir@/gnome-session-binary --systemd-service --session=%i
 ExecStopPost=-@libexecdir@/gnome-session-ctl --shutdown