fix: do not shut down executor when WatchDog is shut down

gax/src/main/java/com/google/api/gax/rpc/Watchdog.java

@@ -124,7 +124,6 @@ public void run() {
   @Override
   public void shutdown() {
     future.cancel(false);
-    executor.shutdown();
   }
 
   @Override

gax/src/test/java/com/google/api/gax/rpc/WatchdogTest.java

@@ -188,7 +188,6 @@ public void testWatchdogBeingClosed() {
         .scheduleAtFixedRate(
             underTest, checkInterval.toMillis(), checkInterval.toMillis(), TimeUnit.MILLISECONDS);
     Mockito.verify(future, Mockito.times(2)).cancel(false);
-    Mockito.verify(mockExecutor, Mockito.times(2)).shutdown();
 
     underTest.shutdownNow();
     Mockito.verify(future).cancel(true);