Remove the Descriptor Published field

From ticket:12692#comment:9

I find "Published: Never" confusing.

Others as well: ​https://lists.torproject.org/pipermail/tor-relays/2017-March/012048.html

What onionoo field is used for that?

This field isn't backed by any data. A quick grep only turns up that field in the template and no assignments in the model. Onionoo also doesn't mention desc_published.