Hi, i'd like to use "sudo su" command (not "sudo gainroot" with R&D mode on) but I don't know the root password, so I install sudser.

Is there a way to remove root password for user without install any deb?

After enabling RD mode (and removing sudser), I use "passwd -d root" but when I type "sudo su" appears a message and no root shell

what message?
"Sorry, user user is not allowed to execute '/bin/su' as root" ?
if that so, you might want to make a new entry at /etc/sudoers.d/ for "su". just copy from any existing one and make your own changes there

Commands:

As user:
sudo gainrooot

As root:
passwd -d user
passwd -d root
reboot

wait...

As user:
sudo su

Output -> Sorry, user user is not allowed to execute '/bin/su' as root on Nokia-N900

This seems nonsense to me, why do you need to run sudo su?
What do you expect to obtain?

1) use su with root password to open a root shell
2) use sudo <command> to perform a command as root (if allowed in /etc/sudoers)
3) use ssh root@localhost to open a root shell (with root password)

Don't delete your root password as this exposes you to all sorts of security risks.