Much faster and safer solution.

Boot OSX

Open the terminal

mount boot partition

mkdir mnt
sudo mount -t msdos /dev/disk0s1 mnt

You will see a new drive "EFI", open this drive and open the folder "EFI". Inside you will have the folders "APPLE" and "UBUNTU".

delete ubuntu EFI Boot folder

cd EFI
# Just delete the "UBUNTU".
rm -rf ubuntu

Restart and enjoy!