doc: remove info about the Ubuntu PPA from building docs.
doc: remove info about the Ubuntu PPA from building docs.

Last version there is 2018.04 and it only lures people into installing
an outdated version.

mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples