You can uninstall or removes an installed libgnokii-dev package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove libgnokii-dev
If you would like to remove libgnokii-dev and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove libgnokii-dev
If you use with purge options to libgnokii-dev package all the configuration and dependent packages will be removed.
$ sudo apt-get purge libgnokii-dev
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 libgnokii-dev