Remove mention of outcome in Channel spec

It is mentioned in the "outcome" spec, so premature here.

Author: lizmat

S17-concurrency.pod

@@ -411,14 +411,6 @@ calls to a channel return the same promise, not a new one.
 While C<receive> blocks until it can read, C<poll> takes a message from the
 channel if one is there or immediately returns C<Nil> if nothing is there.
 
-The C<outcome> construct also works on channels, and will try to receive
-values from the any C<Channel> that has value(s) available until they are
-all closed.
-
-    outcome {
-        whenever $channel { take $_ }
-    }
-
 This is such a common pattern that we make a channel in list context behave
 that way: