Massively simplify API binding logic

Changes the API internals to use normal Laravel binding which automatically supports nested-models and can determine their relationships. This removes a lot of confusingly complex internal logic and replaces it with standard Laravel code.

This also removes a deprecated "getModel" method and fully replaces it with a "parameter" method that does stricter type-checking.

app/Http/Requests/Api/Application/Users/DeleteUserRequest.php

@@ -2,7 +2,6 @@
 
 namespace Pterodactyl\Http\Requests\Api\Application\Users;
 
-use Pterodactyl\Models\User;
 use Pterodactyl\Services\Acl\Api\AdminAcl;
 use Pterodactyl\Http\Requests\Api\Application\ApplicationApiRequest;
 
@@ -17,14 +16,4 @@ class DeleteUserRequest extends ApplicationApiRequest
      * @var int
      */
     protected $permission = AdminAcl::WRITE;
-
-    /**
-     * Determine if the requested user exists on the Panel.
-     */
-    public function resourceExists(): bool
-    {
-        $user = $this->route()->parameter('user');
-
-        return $user instanceof User && $user->exists;
-    }
 }