btrfs: Remove btrfs_file_private::trans

Now that the userspace transaction IOCTL have been removed, this member
is no longer used so just remove it
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1268,7 +1268,6 @@ struct btrfs_root {
 };
 
 struct btrfs_file_private {
-	struct btrfs_trans_handle *trans;
 	void *filldir_buf;
 };