How to uninstall libavformat56 from Ubuntu
To uninstall just libavformat56 package itself from Ubuntu execute on terminal:
sudo apt-get remove libavformat56
Uninstall libavformat56 and it's dependent packages
To uninstall the libavformat56 package and any other dependant package which are no longer needed on Ubuntu.
sudo apt-get autoremove libavformat56
If you also want to delete configuration and/or data files of libavformat56 from Ubuntu then this will work:
sudo apt-get purge libavformat56
To delete configuration and/or data files of libavformat56 and it's dependencies from Ubuntu then execute:
sudo apt-get autoremove --purge libavformat56
libavformat56 package information
Libav file format library
More information about apt-get remove
Advanced Package Tool, or APT, is a free software user interface that works with core libraries to handle the installation and removal of software on Debian, Ubuntu and other Linux distributions. APT simplifies the process of managing software on Unix-like computer systems by automating the retrieval, configuration and installation of software packages, either from precompiled files or by compiling source code.
apt-get is the command-line tool for handling packages, and may be considered the user's "back-end" to other tools using the APT library.
apt-get remove is identical to install except that packages are removed instead of installed. Note that removing a package leaves its configuration files on the system. If a plus sign is appended to the package name (with no intervening space), the identified package will be installed instead of removed.