You can uninstall or removes an installed gnumach-image-1.3.99-486-dbg package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove gnumach-image-1.3.99-486-dbg
If you would like to remove gnumach-image-1.3.99-486-dbg and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove gnumach-image-1.3.99-486-dbg
If you use with purge options to gnumach-image-1.3.99-486-dbg package all the configuration and dependent packages will be removed.
$ sudo apt-get purge gnumach-image-1.3.99-486-dbg
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 gnumach-image-1.3.99-486-dbg