Fix transport manager shutdown queuing

src/objective-c/GRPCClient/private/GRPCTransport+Private.m

@@ -46,11 +46,10 @@
   return self;
 }
 
+// Must be called on _dispatchQueue or queues targeted by _dispatchQueue
 - (void)shutDown {
-  dispatch_async(_dispatchQueue, ^{
-    self->_transport = nil;
-    self->_previousInterceptor = nil;
-  });
+  _transport = nil;
+  _previousInterceptor = nil;
 }
 
 - (dispatch_queue_t)dispatchQueue {