Remove unused config param: debug.screen_id

The `debug.screen_id` is a remnant of the `1.2` version, that brought its own `saveScreenshot` version: https://github.com/sanpii/behatch-contexts/blob/1.2/src/Sanpi/Behatch/Context/DebugContext.php#L45-L58

Now the `DebugContext` uses the Mink built-in `saveScreenshot` method, and no longer use `import`, but the configuration and its validation still keep a trace of `screen_id`, which is no longer used.

This PR removes all traces of this unused configuration argument.