virtio_blk: remove virtblk_update_cache_mode

virtio_blk: remove virtblk_update_cache_mode

virtblk_update_cache_mode boils down to a single call to
blk_queue_write_cache.  Remove it in preparation for moving the cache
control flags into the queue_limits.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>