You should use the following steps to hide GRUB (it is still installed)

1. Run this command in terminal

   
   
   

   sudo -H gedit /etc/default/grub
   
   




2. Set these lines (create them if they don't exist), doesn't have to be this order

   
   
   

   GRUB_TIMEOUT=0
   
   GRUB_HIDDEN_TIMEOUT=0
   
   GRUB_HIDDEN_TIMEOUT_QUIET=true
   
   GRUB_DEFAULT=0
   
   




3. Save then close the file




4. Run this command in terminal

   
   
   

   sudo update-grub