removed checks of not existing class Zend\Math\BigInteger

This addresses the last note of #3512 
The second note seems  to be fixed already

but the first one is already there and I cant fix:
```
#grep -iHrn 'ConstraintKeyObject' library/Zend/Db/
../library/Zend/Db/Metadata/Source/AbstractSource.php:398:                $keys[] = $key = new Object\ConstraintKeyObject($constraintKeyInfo['column_name']);
```

@ralphschindler @Maks3w ping