Hide association input for deeply nested fields

This allows deeply nested fields to behave the same as nested fields  in a form. If there's an inverse_of option on the association, do not show the association input in the nested field.

Here's an example:

```
class Field
  has_many :nested_fields, inverse_of: :field
  accepts_nested_attributes_for :nested_fields
end

class NestedField
  belongs_to :field, inverse_of: :nested_field
  has_many :deeply_nested_fields, inverse_of: :nested_field
  accepts_nested_attributes_for :deeply_nested_fields
end

class DeeplyNestedField
  belongs_to :nested_field, inverse_of: :deeply_nested_field
end
```

In the past, on the form for field, you could add nested fields and deeply nested fields.  However, the nested form for nested fields would not should the input option for field, but the deeply nested form for deeply nested fields would show the input option for nested fields.  This pull request fixes the latter case so that it automatically sets the nested field on the deeply nested field and does not show the input.