Also drop the now unused readlink_copy export.

[dchinner: use d_inode(dentry) rather than dentry->d_inode]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>