Don't encourage usage of `columns_hash`

As discussed in https://github.com/plataformatec/simple_form/pull/1094,
we should not encourage usage of `columns_hash`, and instead provide an
alternate method to determine whether or not an attribute exists.

The language `attribute` was chosen over `column` since these are in the
`AttributeMethods` module.
This message might be too technical for the average user, with terms like "null object" and such. (The ordinary developer would probably just read it as "returning `nil`" here.)

Maybe something like...

```
You are seeing this deprecation message because you called `column_for_attribute` with the attribute name "#{name}", which does not have a corresponding database column. In Rails 5.0, calling `column_for_attribute` with a non-existent column name will return a special column object instead of `nil`. If you are using this method to check for an attribute's existence, you should use `attribute_exists?` instead.
```

It could be done in a separate PR though. @sgrif does that seem good to you?