Perhaps more sensible than getting rid of the WIN boot prompt, is to have grub default to booting whatever it booted last time
sudo gedit /etc/default/grub
Find the line that says
and change it to
Save the file. Now update grub
At the next boot you'll have to choose ubuntu, but thereafter it'll always select to boot to whatever it booted last time (ubuntu in this case), unless you choose another option within the 5sec (or whatever it is) that you get to choose.