Remove automatic pages legacy storage migration

Remove these 2 files and specs for them:

• https://gitlab.com/gitlab-org/gitlab/blob/50d70ae5a15581c911d164ef2a93b98c0b1bfbf8/db/post_migrate/20210302150310_schedule_migrate_pages_to_zip_storage.rb#L3
• https://gitlab.com/gitlab-org/gitlab/blob/50d70ae5a15581c911d164ef2a93b98c0b1bfbf8/lib/gitlab/background_migration/migrate_pages_to_zip_storage.rb#L10