Cleanup columns related to delayed project/group deletion

Now that delayed deletion is the default for paid projects/groups, we can safely drop the columns we were using to manage delayed project/group removal at the instance level and at the group level.

At the instance level, these columns can be dropped:

• ApplicationSetting#delayed_project_removal
• ApplicationSetting#lock_delayed_project_removal
• ApplicationSetting#delayed_group_deletion

At the group level, these columns can be dropped:

• NamespaceSetting#delayed_project_removal
• NamespaceSetting#lock_delayed_project_removal

We should also remove the usages these columns in the code.