Stable6 OC_DB::rollback()

There is a `\OCP\DB::beginTransaction()`
There is a `\OCP\DB::commit()`
**But there is no** `\OCP\DB::rollback()` :collision:

https://github.com/owncloud/core/blob/master/lib/public/db.php
https://github.com/owncloud/documentation/blob/2ec72ebbb28c7bd0b9f41bbf088c1ee53953eaca/developer_manual/app/app/database.rst

--
    ownCloud 6.0.4 (stable)
    php 5.4.4-14+deb7u12
    MySQL 5.5.37-0+wheezy1
    debian 7.6 (wheezy) 32 bits