Remove old compatibility code

[Windvis]

It seems the addon still contains code for very old (Ember Data) addon compatibility:

For example:

ember-test-helpers/addon/addon-test-support/@ember/test-helpers/-internal/build-registry.ts

Lines 150 to 158 in dd7bd45

	if (has('ember-data/setup-container')) {
	// ember-data is a proper ember-cli addon since 2.3; if no 'import
	// 'ember-data'' is present somewhere in the tests, there is also no `DS`
	// available on the globalContext and hence ember-data wouldn't be setup
	// correctly for the tests; that's why we import and call setupContainer
	// here; also see https://github.com/emberjs/data/issues/4071 for context
	let setupContainer = require('ember-data/setup-container')['default'];
	setupContainer(owner);
	}

I verified in the Discord and this code should no longer be needed.

A benefit of removing this code is that it still uses loader.js code directly, which is up for removal: emberjs/rfcs#938

[NullVoxPopuli]

This will be removed in the next major!