Step 0 :
update-manager -cd
After that, reboot the box.
Step 1 :
sudo apt remove gnome-session ubuntu-gnome-desktop gnome-session-flashback plymouth-theme-ubuntu-gnome-text plymouth-theme-ubuntu-gnome-logo gnome-session-flashback
select "gdm3" when asked.
Step 2 :
sudo update-alternatives --config default.plymouth
select "auto mode" of "/usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth"
Step 3 :
sudo update-initramfs -u
Then reboot.
That's all! See you.