NodePreviewController should not render entity links in the HTML head

Problem/Motivation

This was found in #2542834: Menu item translation : breadcrumb displays (twice) "add" that have no sense.

NodePreviewController contains code - copy-pasted from NodeViewController - to render entity links in the HTML head.

For proper node views this is fine, but for previews this is not only pointless but also problematic because at this point the entity does not have an ID, so that the links actually cannot be generated properly. It only works because $entity->url() works around this behavior explicitly. If we were to use $entity->toUrl(), NodePreviewController would blow up.

Proposed resolution

Remove the link entirely for the node preview.

Remaining tasks

User interface changes

None.

API changes

None. Although I guess it would be nice to write a change notice for this.

Data model changes

None.