You can uninstall or removes an installed ipmi-modules-4.4.0-21-powerpc- package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove ipmi-modules-4.4.0-21-powerpc-
If you would like to remove ipmi-modules-4.4.0-21-powerpc- and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove ipmi-modules-4.4.0-21-powerpc-
If you use with purge options to ipmi-modules-4.4.0-21-powerpc- package all the configuration and dependent packages will be removed.
$ sudo apt-get purge ipmi-modules-4.4.0-21-powerpc-
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 ipmi-modules-4.4.0-21-powerpc-