btrfs: drop btrfs_device::can_discard to query directly

We can query the bdev directly when needed at btrfs_discard_extent()
so drop btrfs_device::can_discard.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Suggested-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>