Remove the `System::getKernel()` method

This PR removes the `System::getKernel()` method as discussed. You can still get the kernel using one of the following:

```php
// Use the container
$kernel = System::getContainer()->getKernel();

// Use the global variable
global $kernel;
```

The first method assumes that the container is available, which however is not the case during the bundle autoloading process. I have therefore used the global variable variant.

@contao/developers Please vote :)