virtio-blk: remove a spurious call to revalidate_disk_size

revalidate_disk_size just updates the block device size from the disk
size.  Thus calling it from virtblk_update_cache_mode doesn't actually
do anything.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>