Btrfs: remove unused variables in __btrfs_unlink_inode

This code was first introduced in 5f39d397 ("Btrfs: Create
extent_buffer interface for large blocksizes") and the function was
named btrfs_unlink_trans. It later got renamed to __btrfs_unlink_inode
and finally commit 16cdcec7

 ("btrfs: implement delayed inode items
operation") changed the way inodes are deleted and obviated the need for
those two members.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ replace changelog by Nikolay's version ]
Signed-off-by: David Sterba <dsterba@suse.com>