The uninstall method is pretty much constructed the same way as the install method.

First, create an uninstall.sql file in the install directory of your module in which you will write the following SQL command to drop the mymod_comment table:

DROP TABLE `PREFIX_mymod_comment`;

Next, in mymodcomment.php, write an uninstall method to do the following:

We will also delete the configuration values MYMOD_GRADES and MYMOD_COMMENTS. It isn't mandatory but it's cleaner that way.

At the end, you should have something like this:

public function uninstall()
{
  // Call uninstall parent method
  if (!parent::uninstall())
    return false;

  // Execute module install SQL statements
  $sql_file = dirname(__FILE__).'/install/uninstall.sql';
  if (!$this->loadSQLFile($sql_file))
    return false;

  // Delete configuration values
  Configuration::deleteByName('MYMOD_GRADES');
  Configuration...