[5.5] Model::$exists to be set to false when force-deleting a model using SoftDeletes

[PGBI]

On model force deletion, made sure that $model->exists is set to false.

[taylorotwell]

Wasn't this recently changed? I feel like it keeps going back and forth.

[PGBI]

Sorry, I should have declined and resubmitted.

In the initial PR, I made $this->exists = false; for both soft deletion and force deletion. But it caused this new bug:

$softDeletableModel->delete(); // model is soft deleted, exists == false
$softDeletableModel->forceDelete(); // <--- nothing happens, because delete() would abort if $exists == false

So I went back to setting $this->exists = false; on force deletion only.