Implement ChildNode.remove()

[hmac]

Fixes #2191

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1346

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[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @kmcallister (or someone else) soon.

[Ms2ger]

Commented on critic; please leave your email address there.

[jdm]

Waiting on further review by @Ms2ger.

[Ms2ger]

@Hazz, please rebase as explained at https://github.com/mozilla/servo/wiki/Github-%26-Critic-PR-handling-101

[Ms2ger]

@Hazz, could you rebase again? I'll try to make sure it's the last time :)

[jdm]

I'm rebasing this; don't worry about it.

[jdm]

Rebased in #2326.