fix: Watchdog.shutdownNow() does not shutdown executor

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

@@ -151,7 +151,6 @@ public boolean isTerminated() {
   @Override
   public void shutdownNow() {
     future.cancel(true);
-    executor.shutdownNow();
   }
 
   @Override

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

@@ -191,7 +191,7 @@ public void testWatchdogBeingClosed() {
 
     underTest.shutdownNow();
     Mockito.verify(future).cancel(true);
-    Mockito.verify(mockExecutor).shutdownNow();
+    Mockito.verifyNoMoreInteractions(mockExecutor);
   }
 
   static class AccumulatingObserver<T> implements ResponseObserver<T> {