Freeze exception request for removing extra packages in oem-config

Binary package hint: ubiquity

I've implemented the final outstanding item in <https://blueprints.launchpad.net/ubuntu/+spec/foundations-lucid-oem-config>, removing packages in oem-config that were installed by the distributor before running oem-config-prepare, but are not needed for the final system.

The most common use case for this is installing all available language packs and removing all but the language selected in oem-config, so that the user doesn't need a network connection to get adequate language support but doesn't get left with hundreds of megabytes of unnecessary packages.

As this is guarded by a preseed question (oem-config/remove_extras) that's false by default, this should be a very safe change.

I've tested this by installing vim, gnome-do, and a lot of language packs, then selecting Spanish and checking that everything but the Spanish langpacks were removed.