Delete - Ecto.Adapters.SQL.Connection

[Danwhy]

Part of #38

https://hexdocs.pm/ecto_sql/Ecto.Adapters.SQL.Connection.html#c:delete/4

We'll need to implement this callback ourselves as we need to ensure data is not actually deleted, but only marked as so. It's unlikely that we'll be able to use any of the existing functionality.