The default PHP and phpMyAdmin packages are in the ubuntu repos, which are enabled on your Droplet. When you installed that PPA, it was set as a higher priority than the default repos so that its packages are installed instead.
One option would be removing the PPA using
ppa-purge which will gracefully downgrade the packages to those of the default repos. Keep in mind however that if you're downgrading to a PHP version that is much older than what you currently have, your configs might not be compatible with it, and you might have to adjust them accordingly.
sudo apt-get install ppa-purge
Then, remove the
sudo ppa-purge ppa:ondrej/php
If you only want to remove the PPA without affecting any installed packages, however, you can simply run the
add-apt-repository command again but with the
sudo apt-add-repository --remove ppa:ondrej/php
Finally, as with any other change to the list of apt repositories on your system, run
apt-get update so that apt's repo cache matches the updated list.
sudo apt-get update