Changed Comment Module and database table - how to revert changes?
First: Do not hack core!

To restore your site to a stable state, revert to the default theme, back-up your database and the sites/ sub-directory and anything below it.  Then download a fresh copy of the core and install it. Finally, restore the sites/ sub-directory, your database, and the theme.

Having extra tables in the database that Drupal does not know about does not do any harm, but you may drop those tables that you created yourself before backing up the database.