Removed a broken failure condition from the "script" of mission 13.

To my recollection, actually, this bug has *always* been present.
At first, I fixed it (the problem was the entry in script13.txt
was malformed), but when I did that, it caused the objective of
destroying Ursula's ship to be considered completed. In other words,
the game doesn't even have code to handle this as a failure
condition! So I've decided to remove it instead. It wasn't that
great of an idea, anyway.

data/script13.txt

@@ -15,17 +15,3 @@ FACE_PHOEBE
 -1
 0
 Just try and dodge them!
-
-180
-FACE_SID
--1
-0
-Chris! Ursula's running away!
-
-195
--1
-0
-14
-128
-
-0