Given a model with multiple `belongs_to` associations where the inverse
associations are `has_one`s, and all associations have `dependent:
:destroy`, destroying any of the `has_one` models should destroy the
`belongs_to` model and all its associated `has_one` models.

However this only worked from one end.  When destroying the
"non-working" end, the `belongs_to` model was correctly destroyed but
the other `has_one` end was not.

Here is an example using `has_one :through`:

    class Left < ActiveRecord::Base
      has_one :middle, dependent: :destroy
      has_one :right, through: :middle
    end

    class Middle < ActiveRecord::Base
      belongs_to :left, dependent: :destroy
      belongs_to :right, dependent: :destroy
    end

    class Right < ActiveRecord::Base
      has_one :middle, dependent: :destroy
      has_one :left, through: :middle
    end

In this example:

- `right.destroy` correctly destroys its `middle` and its `left`;
- `left.destroy` destroys its `middle` but not its `right`.

The end which worked depended on the order of `belongs_to` statements in the
`belongs_to` model.

This commit ensures that the no matter which `has_one` model you destroy, all
associated models are destroyed as expected.

Fixes #50948.