Remove unused variable

Author: jmaslak

doc/Type/IO/Socket/Async.pod6

@@ -162,7 +162,6 @@ Or when using C<whenever>:
 
 =begin code
 my $listener = IO::Socket::Async.listen('127.0.0.1', 5000);
-my $big-finish = Promise.new;
 my $tap;
 react {
     $tap = do whenever $listener -> $conn { ... }