Remove google code as an import source

What does this MR do and why?

This change removes google code as an import source from the Gitlab::ImportSources module.

Support for google code as an import source was removed over two years ago, but had not been removed from the ImportSources module, so was still appearing in the Admin > Settings > General > Visibility and Access Controls > Import Sources list.

Screenshots or screen recordings

Before:

After:

How to set up and validate locally

For UI changes:

1. check out the branch
2. if not already started, start the gdk: gdk start
3. visit Admin > Settings > General > Visibility and Access Controls > Import Sources
4. confirm google code is not in the list of import sources

For DB changes:

1. in the terminal run the migration: bin/rails db:migrate RAILS_ENV=development
2. enter the rails console gdk rails console
3. find current import_sources as stored on the main DB ApplicationSetting.current.import_sources
4. ensure "google_code\" does not appear in the result.
5. in the terminal rollback the migration bin/rails db:rollback:main STEP=1 RAILS_ENV=development
6. in the rails console find current import_sources again ``ApplicationSetting.current.import_sources`
7. ensure "google_code\" appears in the result

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #379715 (closed)