You can uninstall or removes an installed lubuntu-nexus7-extra-files package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove lubuntu-nexus7-extra-files
If you would like to remove lubuntu-nexus7-extra-files and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove lubuntu-nexus7-extra-files
If you use with purge options to lubuntu-nexus7-extra-files package all the configuration and dependent packages will be removed.
$ sudo apt-get purge lubuntu-nexus7-extra-files
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 lubuntu-nexus7-extra-files