I'm not able to locate the file inittab in order to unable auto-login on serial with my new RasPI. (I tried at /etc but no file !)
The best I can do is give you a hint since I haven't done auto-login on serial.
inittab was part of sysvinit, the latest versions of Debian and Raspbian (viz., Jessie) use systemd which doesn't have inittab. Instead it uses systemctl to control the things that inittab did before.
do a man on systemctl to maybe find what you're after.