Bird's ability to name sessions is redundant

``` php
$bird = new \Gajus\Skip\Bird([session name]);
```

Bird's constructor parameter allows to define bird's scope. However, this is redundant because `session_name` can be used instead.

Start at the Bird constructor shown in the issue and inspect how its session scope is used alongside session_name. Confirm the intended API change and identify any documented or tested constructor behavior before deciding what would constitute removing the redundant naming capability.

Written by the indexing model from the issue text.