an outdated comment for hash_seq_init.

Hello.

While I looked into dynahash.c, I found that the following
sentense became outdated by 5dfc198. The commit removed the only
usage of hash_freeze().

> * NOTE: it is possible to use hash_seq_init/hash_seq_search without any
> * worry about hash_seq_term cleanup, if the hashtable is first locked against
> * further insertions by calling hash_freeze.  This is used by nodeAgg.c,
> * wherein it is inconvenient to track whether a scan is still open, and
> * there's no possibility of further insertions after readout has begun.

The attached patch removes the sentence begins with "This is used
by..".

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center