test: fix flaky test-gc-net-timeout

There's a global.gc() invoked in an interval, and a second one in a
req.setTimeout() callback. Remove the one in the callback. I'm not sure
how competing global.gc() calls might result in a deadlock, but it seems
plausible and empirical testing confirms that it makes the test
reliable.

Fixes: nodejs#23067

test/sequential/test-gc-net-timeout.js

@@ -41,7 +41,6 @@ function getall() {
   req.setTimeout(10, function() {
     req.destroy();
     done++;
-    global.gc();
   });
 
   count++;