Reinstall LightDM

# apt-get purge lightdm
# apt-get install lightdm
# dpkg-reconfigure lightdm

# /usr/lib/lightdm/lightdm-set-defaults -u ubuntu  ------------  Set user-session
# /usr/lib/lightdm/lightdm-set-defaults -g unity-greeter -------  Set greeter-session
# /usr/lib/lightdm/lightdm-set-defaults -l false   ------------  Disable guest login

# cat /etc/lightdm/lightdm.conf
[SeatDefaults]
user-session=ubuntu
greeter-session=unity-greeter
allow-guest=false
#