depracated userClass on Application

$userClass causes error:
```sh
Uncaught TypeError: Cannot assign null to property kingston\icarus\Application::$userClass of type string in /home/kingston/Documents/icarus-project/base-application/vendor/kingston/icarus/Application.php:160
Stack trace:
#0 /home/kingston/Documents/icarus-project/base-application/public/index.php(29): kingston\icarus\Application->__construct()
#1 {main}
thrown in /home/kingston/Documents/icarus-project/base-application/vendor/kingston/icarus/Application.php on line 160
```

to fix this the userClass is no longer required to start the application

Start in vendor/kingston/icarus/Application.php around line 160 and trace construction from public/index.php line 29. Reproduce startup without a userClass value, then verify the application starts without the reported null assignment error.

Written by the indexing model from the issue text.