Publicize: remove link from Facebook update for "Post" update types.

1. Connect Publicize to a Facebook Page.
2. Install, configure, and connect [the SNAP plugin](https://wordpress.org/plugins/social-networks-auto-poster-facebook-twitter-g/) to that same Facebook Page.
3. Configure and Connect the WordPress and Facebook channels on [IFTTT](https://ifttt.com/wordpress).
4. Publish a new post.

Here is how the Publicized post looks like:


Here is how it looks like when posted via SNAP:


And here is how it looks like when posted via IFTTT:


A few things differ here:
1. Publicize added an automated post excerpt as I didn't specify a custom Publicize message. That's the expected behaviour.
2. Both SNAP and IFTTT allow you to customize the default template used for the message. That's something we'll address in #261, as Publicize would benefit from such templates and placeholders.
3. **Both SNAP and IFTTT remove the post URL from the message after passing it to Facebook. That's something we don't do, but it appears we could do**, and it wouldn't stop Facebook from building the post preview. We just have to keep in mind that the link should remain for Image, Gallery, and Video update types, where there is no post preview below the message.

Start at the Publicize handling for Facebook "Post" update types and trace how the post URL is added to the message. Reproduce the issue using the Publicize, SNAP, and IFTTT setup described above, then verify that Post messages omit the URL while Image, Gallery, and Video messages retain it.

Written by the indexing model from the issue text.