Remove ActiveRecord::AttributeAssignment#assign_nested_parameter_attributes

[seanpdoyle]

Motivation / Background

The private #assign_nested_parameter_attributes was introduced in 774ff18 (Dec 15, 2011), moved to ActiveRecord::AttributeAssignment in ceb33f8 (also Dec 15, 2011), then most recently modified in 2606fb3 (Jan 23, 2015) when ActiveModel::AttributeAssignment was extracted.

Support for accepts_nested_attributes_for (introduced in ec8f045 Feb 1, 2009) pre-dates those commits, and has evolved enough to cover this behavior in other ways.

Detail

With its removal, Active Record's test suite still passes, so if it's a crucial piece of code to retain, we should expand the suite to exercise it.

Additional information

The fact that removing these methods doesn't fail the test suite came out of #49675. If that PR isn't deemed to be viable, removing the code in this PR might still have value. If that PR is deemed to be viable, we might want to merge this PR ahead of it, then make similar changes to remove the calls from https://github.com/rails/rails/pull/49675/files#diff-bd47ea9eef412ec5aca24908af217592cef01d24fd54eeec976bd37bd9a18151R80.

[p8]

The following test was added in 774ff18 to ensure the assignment was working as expected:

rails/activerecord/test/cases/nested_attributes_test.rb

Lines 620 to 623 in 774ff18

	def test_should_take_a_hash_with_owner_attributes_and_assign_the_attributes_to_the_associated_model
	@pirate.birds.create :name => 'bird', :pirate_attributes => {:id => @pirate.id.to_s, :catchphrase => 'Holla!'}
	assert_equal 'Holla!', @pirate.reload.catchphrase
	end

This test was removed in #4122 as it was failing and probably broken because of mass assignment protection.
There is a discussion in the PR.

Enabling the test again, will cause it to fail in 6-0-stable and main.
So I think we can remove assign_nested_parameter_attributes.

[p8]

It also seems to me that assign_nested_parameter_attributes calls _assign_attribute(key, v), which is the same as _assign_attributes does in the else statement. It's only done after all other key/value pairs have been set.

[seanpdoyle]

@rafaelfranca is the historical context provided in #49678 (comment) sufficient to consider this code dead and safe to remove?