You can uninstall or removes an installed claws-mail-attach-warner package itself from Ubuntu 16.10 (Yakkety Yak) through the terminal,
$ sudo apt-get remove claws-mail-attach-warner
If you would like to remove claws-mail-attach-warner and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove claws-mail-attach-warner
If you use with purge options to claws-mail-attach-warner package all the configuration and dependent packages will be removed.
$ sudo apt-get purge claws-mail-attach-warner
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 claws-mail-attach-warner