Btrfs: remove dead comments for read_csums()

Chris introduced hleper function  read_csums() and this function
has been removed, but we forgot to remove its corresponding comments.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>