I quite enjoyed what happened here 
LightDM GTK Greeter Settings will save setting in /etc/lightdm/lightdm-gtk-greeter.conf and has no options for autologin true/false or on/off because the settings for that are in /etc/lightdm/lightdm.conf
So, remove your name from the line 126 to be like this
autologin-user=
or comment the line
#autologin-user=