Migration lib update for easy use of MY_Migration

Update: removed the code from **CI_Migration** that skipped the *__construct()* execution when **MY_Migration** was present.

Reasoning:
* The user can avoid the original *__construct()* in **MY_Migration** by not calling *parent::__construct()* in his own constructor.
* On the other hand, an user who did not intend any modification on **CI_Migration::__construct()** in his **MY_Migration** library was forced to copy-paste the original constructor in his own library just to simulate the default functionality.