Removed useless test in misc/cron/archive.php

In the past, `misc/cron/archive.php` used to be included in the console command `core:archive`. But not anymore, so this test can be removed.
The console now includes the optional file bootstrap.php

Users can add a custom `bootstrap.php` file at the root of Piwik to customize constants. This file is included (if exists) in `index.php` and `piwik.php` but not in the console. This is now fixed.

See the user docs about this: http://piwik.org/docs/include-piwik-in-your-project/
Moved common bootstrap code in a `core/bootstrap.php` file

This file is not in a class because it needs to be compatible with PHP 4.
Removed Piwik\Loader: Composer's autoloader is now included in core/bootstrap.php
Added a comment
Merge pull request #6891 from piwik/bootstrap

Simplified bootstrap files