rhashtable: remove outdated comments about grow_decision etc

grow_decision and shink_decision no longer exist, so remove
the remaining references to them.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>