Cleanup some private API and now-unsupported usage

[NullVoxPopuli]

Private APIs I'm not removing, but are tested in ember.js, so we don't accidentally break:

• Owner-related Mixins: https://github.com/emberjs/ember.js/blob/aedf98854d89887f5779a3ee0d4f0ff2a26a8039/packages/ember/tests/reexports_test.js#L344-L345
• Registry: https://github.com/emberjs/ember.js/blob/aedf98854d89887f5779a3ee0d4f0ff2a26a8039/packages/ember/tests/reexports_test.js#L283
• getOnerror / setOnerror: https://github.com/emberjs/ember.js/blob/aedf98854d89887f5779a3ee0d4f0ff2a26a8039/packages/ember/tests/reexports_test.js#L302

Note:

• we should have a public API way to build an owner so we can have @ember/test-helpers not use private APIs.
  • would allow us to not use the Mixins
• I can delete more, but it would technically be a breaking change for this library, I think
  • we have support for custom registries, custom owners, etc... I don't think this makes sense