Remove google code as an import source.

Support for google code import was removed a couple of years ago but it wasn't removed as an import source from the Gitlab::ImportSources module.

We need to remove this so that it no longer appears as an import source in the Admin > Seetings > General > Visibility and access controls > Import sources list.

We'll also need to remove reference to it in lib/api/settings.rb and spec/lib/gitlab/import_sources_spec.rb