btrfs: Remove btrfs_fs_info::open_ioctl_trans

Since userspace transaction have been removed we no longer have use
for this field so delete it.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>