Deletes the segment and answers 204 No Content with no body. Its place in the plan's saved
segments is freed at once, so a save refused with 402 quota_exceeded can be sent again.
An export already submitted goes on and writes the segment as it was admitted; its job
keeps a copy under segment.
Every watchlist that follows the segment stops following it, and its unit of the
watched-segments allowance is freed. Items already delivered stay in the
feed, with segment.name null from then on.
A segment is not edited in place: to change its filters or name, delete it and save a new
one, which gets a new id, and follow the new one again if a list followed the old: it
starts from the companies in it then, and delivers only those that enter afterwards. A segment of another organization and an id that never existed
are both 404 not_found. Deleting is not undone. Not counted as a call.