Clean up the magic properties in $node for content migrations

Now that #1733658: Inject a comment body text area and input format selector into the node edit form is committed, there's already a place you can put the comment body and input format selection into $node so that it'll be used when generating the comment for a given revision, namely $node->nodechanges_comment_body['value'] and $node->nodechanges_comment_body['format']. Therefore, I'd like to propose we rip out the support for $node->nodechanges_body and $node->nodechanges_format and teach the migration code that's relying on that to use the better-namespaced ones, instead.

However, this isn't blocking launch, so I'm not even tagging for Drupal.org D7. Just wanted to capture this into an issue so it doesn't get lost when I close #1733658.