Atomic Downgrade: Allow users to delete Atomic sites

[mmtr]

Let's provide customers the ability to delete their Atomic sites without HE intervention. To that end, once users are able to cancel/remove plans from Atomic sites (#55194), we should remove the restrictions that are currently in place in "Settings > General > Delete your site immediately".

Additionally, we could also add an extra option to the revert warning (#55239) so the lossless import can be skipped and therefore users won't nee to wait +10 minutes in they simply want to delete the site after cancelling/removing the plan.