Infrastructure: Remove .pr-preview.json (pull #2426)

Fix #2416

This removes `.pr-preview.json` which seems to allow `pr-preview[bot]` to add a link to the editor's draft (which is no longer being used) onto the parent comment of some PRs.

It also seems to be [overwriting the generated APG redesign link](#2416 (comment)) which is attached to PRs by the `michael-n-cooper-bot` bot. An example exists in the edit history for #2415 parent comment.