Rename `Jam_Model::deleted()` to `Jam_Model::is_deleted()`

`is_deleted()` is a more appropriate name as it indicates it would return a boolean.

It also differentiates the method more from `Jam_Model`::delete()`.

In the paranoid behavior there is a builder extension called `deleted()`. Thus `deleted` is not an appropriate name for a model method.

I have checked the documentation for any occurances of `Jam_Model::deleted()` method. It is not documented. And this would be a good time to document `Jam_Model::is_deleted()`.

I have also updated `Jam_Association_Collection` where is the single occurrence of `Jam_Model::deleted()`.

The `Jam_Model::deleted()` method is also not unit-tested.