Remove PR build warning

[ericholscher]

The Files Changed UI solves this problem in a much nicer way.
I think we can get rid of this warning,
since it's just in the way now.

[ericholscher]

Ping :)

[humitos]

We discussed this already in a few other issues/PRs and there are still valid points to not remove it, IMO:

• We still have the option show_on_external in the settings page
• People with that option marked will loose this notification
• The visual diff bar does not communicates the documentation set is from a PR and it's not live (we know that, but users may not)