tutorial: simplify introduction of messaging

The text tries to explain something about `'run`, but it looks like
the sample code has since been updated to wrap `'run` into `a-run` and
`b-run`, so AFAICS this is now just a distraction.

goblins/scribblings/tutorial.scrbl

@@ -701,19 +701,6 @@ objects that are near each other (in the same vat).
 
 Fortunately there's something we can do: we can send a message from
 Bob to Alice.
-But we've never seen message sending in Goblins before, so what is that?
-
-To prototype this, let's use the @id{'run} method on @id{b-vat}.
-Remember, we saw the @id{'run} method used before, where it looked like:
-
-@interact[
-(a-run
- (define alyssa
-   (spawn ^friend "Alyssa"))
- ($ alyssa "Ben"))]
-
-So @id{'run} is just a way to run some arbitrary code in an actor context.
-That sounds good enough for playing around with sending messages.
 We can send messages with @racket[<-] so let's try that:
 
 @interact[