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an outdated version.

doc/building.dox

@@ -232,26 +232,11 @@ and @ref building-integration-packages-msys "Magnum Integration".
 
 @subsection building-packages-deb Packages for Debian, Ubuntu and derivatives
 
-Prebuilt packages of the latest stable version for Ubuntu 14.04, 16.04 and
-18.04 are available through a PPA. To install packages related to Magnum and
-Corrade, you can add the repository and then install the packages with the
-following commands:
-
-@code{.sh}
-sudo add-apt-repository ppa:chrome/magnum.graphics
-sudo apt-get update
-sudo apt-get install magnum magnum-dev
-@endcode
-
-For more information about these packages, please visit
-[the PPA page](https://launchpad.net/~chrome/+archive/ubuntu/magnum.graphics).
-
-To build your own package, the `package/debian/` directory contains all files
-needed for building Debian packages. You need the
-@ref building-corrade-packages-deb "Corrade Debian package" installed and in
-addition also the `dpkg-dev` and `debhelper` packages. Building is easy, just
-change directory to package root, link or copy `package/debian` directory there
-and run `dpkg-buildpackage`:
+The `package/debian/` directory contains all files needed for building Debian
+packages. You need the @ref building-corrade-packages-deb "Corrade Debian package"
+installed and in addition also the `dpkg-dev` and `debhelper` packages.
+Building is easy, just change directory to package root, link or copy
+`package/debian` directory there and run `dpkg-buildpackage`:
 
 @code{.sh}
 git clone git://github.com/mosra/magnum && cd magnum