code: public function dropPrimaryKey($name, $table)
      {
          return 'ALTER TABLE ' . $this->db->quoteTableName($table)
              . ' DROP CONSTRAINT ' . $this->db->quoteColumnName($name);
      }
docs: Builds a SQL statement for removing a primary key constraint to an existing table.
  @param string $name the name of the primary key constraint to be removed.
  @param string $table the table that the primary key constraint will be removed from.
  @return string the SQL statement for removing a primary key constraint from an existing table.