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