How can I completely remove the KDE 5.6 Plasma desktop from Ubuntu 16.04 and restore Unity?

Removing a desktop environment completely is never recommended, since it can break packages that Ubuntu or other DEs depend on. In fact, it's not really recommended that you install a second DE in the first place. If you install a DE, you should really be prepared to either reinstall Ubuntu or just keep the DE installed.

You can use the command below to remove most/all of the Kubuntu/Plasma/KDE packages. Please keep in mind that things can break.

sudo apt-get purge '^kde' '^kubuntu' '^plasma'