Removing ::text casting with strings.

Postgres does not require this cast to happen and in most cases can figure it out on its own without the hint. Other ORMs like Sequel do not do this cast so I feel fine removing it. 

This breaks in cases of non-text fields that 'act' like strings. Example: uuid

```elixir
Repo.get(MyUUIDModel, uuid_key)
(Postgrex.Error) ERROR (42883): operator does not exist: uuid = text
```

Making this change allows it to work.