Remove migration import settings from the settings API

🔥 Problem

The application settings:

• container_registry_import_created_before
• container_registry_import_max_retries
• container_registry_import_max_step_duration
• container_registry_import_max_tags_count
• container_registry_import_start_max_retries
• container_registry_import_target_plan
• container_registry_pre_import_tags_rate

are settings for the Container Registry migration. See &7316 (closed).

This migration is currently set up to only happen on gitlab.com.

In other words, these settings are not useful for self-managed admins nor they have an impact if they are updated (through a rails console for example).

However, these settings are returned in the relevant API.

🚒 Solution

Remove those settings from the API. We should treat them as internal settings that are not exposed to users.