Remove references to the Laravel backup manager

As proposed in https://github.com/CachetHQ/Cachet/pull/3150 the current backup solution produces too many edge cases when configured with the non-default options. Cachet should not be held responsible for backups since it's unable to complete this task properly. Feel free to re-add a laravel backup solution to your custom cachet instance.

config/app.php

@@ -187,7 +187,6 @@ return [
         Laravolt\Avatar\ServiceProvider::class,
         McCool\LaravelAutoPresenter\AutoPresenterServiceProvider::class,
         PragmaRX\Google2FA\Vendor\Laravel\ServiceProvider::class,
-        Spatie\Backup\BackupServiceProvider::class,
         Roumen\Feed\FeedServiceProvider::class,
 
         /*