btrfs: remove dead comment on btrfs_add_dead_root()

The old comment is from the initial merge of btrfs, but since commit
5d4f98a ("Btrfs: Mixed back reference  (FORWARD ROLLING FORMAT
CHANGE)") changed the behavior to not to allocate any extra memory,
the comment on the memory allocation part is out-of-date.

Fix it by removing the dead part and change it to modern behavior.

Signed-off-by: Qu Wenruo <wqu@suse.com>

fs/btrfs/transaction.c

@@ -1316,9 +1316,10 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 }
 
 /*
- * dead roots are old snapshots that need to be deleted.  This allocates
- * a dirty root struct and adds it into the list of dead roots that need to
- * be deleted
+ * Dead roots are old snapshots that need to be deleted.
+ *
+ * This helper will queue them to the dead_roots list to be deleted by
+ * cleaner thread.
  */
 void btrfs_add_dead_root(struct btrfs_root *root)
 {