OAK-765: AbstractNodeState#getChildNode returns null despite @Nonnull return value

Drop the AbstractNodeState.getChildNode() method as nobody is using
the method and returning EmptyNodeState.MISSING would break package
boundaries (Oak SPI should not depend on other non-API classes).

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1466379 13f79535-47bb-0310-9956-ffa450edef68