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>

fs/btrfs/inode.c

@@ -2543,12 +2543,6 @@ out_kfree:
 	return NULL;
 }
 
-/*
- * helper function for btrfs_finish_ordered_io, this
- * just reads in some of the csum leaves to prime them into ram
- * before we start the transaction.  It limits the amount of btree
- * reads required while inside the transaction.
- */
 /* as ordered data IO finishes, this gets called so we can finish
  * an ordered extent if the range of bytes in the file it covers are
  * fully written.