You can uninstall or removes an installed libgtk-dotnet3.0-cil package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove libgtk-dotnet3.0-cil
If you would like to remove libgtk-dotnet3.0-cil and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove libgtk-dotnet3.0-cil
If you use with purge options to libgtk-dotnet3.0-cil package all the configuration and dependent packages will be removed.
$ sudo apt-get purge libgtk-dotnet3.0-cil
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove libgtk-dotnet3.0-cil