remove nslices assertion on windows

src/core/iomgr/tcp_windows.c

@@ -265,7 +265,6 @@ static grpc_endpoint_write_status win_write(grpc_endpoint *ep,
   WSABUF *allocated = NULL;
   WSABUF *buffers = local_buffers;
 
-  GPR_ASSERT(nslices != 0);
   GPR_ASSERT(!tcp->outstanding_write);
   tcp_ref(tcp);