Disallow {fields:{_id:0}} in observeChanges

This implies it is not allowed in `observe` either, or in cursors
returned from publish functions, or in cursors used in {{#each}}

Why? observeChanges and DDP publication use the ID as part of the
callback/message, and eliding it completely breaks them. Meteor UI uses
the ID with {{#each}} to properly move nodes around instead of
re-rendering. We could try to allow it for `observe` outside of
{{#each}}, but it would feel somewhat inconsistent.