storage: fix a bug that table specified index can't be removed
storage: fix a bug that table specified index can't be removed

You can specify table of index by `COMMENT 'table "XXX"'`.

[groonga-dev,02677]

Reported by Naoya Murakami. Thanks!!!

yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga