Remove old storage logic from Import/Export
This should:
- Get rid of the feature flag
- Remove all of the old logic, as at this point we should be using the Uploaders by default.
To be done after: https://gitlab.com/gitlab-org/gitlab-ce/issues/48773 https://gitlab.com/gitlab-org/gitlab-ce/issues/48772 https://gitlab.com/gitlab-org/gitlab-ce/issues/48745 https://gitlab.com/gitlab-org/gitlab-ce/issues/46246