This PR follows [feedback](https://discuss.emberjs.com/t/component-arguments-and-typescript-whats-the-right-approach/19761) from @chriskrycho on Ember Discuss, and aims to reflect the introduction of Component Signatures.

Hopefully this isn't too heavy handed but it seemed to me that previous sections belonged to a time when `args` could be empty object. As defining a Signature is a prerequisite for calling `this.args` it felt right to remove them?