virtio/vsock: remove redundant 'skb_pull()' call

Since we now no longer use 'skb->len' to update credit, there is no sense
to update skbuff state, because it is used only once after dequeue to
copy data and then will be released.

Fixes: 71dc9ec9 ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>