Before #36604, `enum` and `set` columns were incorrectly dumped as a
`string` column.

If an `enum` column is defined as `foo enum('apple','orange')`, it was
dumped as `t.string :foo, limit: 6`, the `limit: 6` is seemed to
restrict invalid string longer than `'orange'`.

But now, `enum` and `set` columns are correctly dumped as `enum` and
`set` columns, the limit as longest element is no longer used.