Remove `unit` proxy method.

Chanko causes an error when a table has `unit` column. I propose with this pull request that we remove `unit` proxy method for the reason below:

  * `unit` proxy method is undocumented
    * Instead, `ext` is documented.
  * `unit` is too generic
  * I guess almost all Chanko users actually don't use this.

What do you think of it?