Fixed overriding default message sources

When overriding i18n translation sources from config using `app*` or `yii*` default `app` and `yii` sources were not removed resulting in using application source language for messages with `app` category.

```php
'i18n' => [
    'translations' => [
        'app*' => [
            'class' => 'yii\i18n\PhpMessageSource',
            'sourceLanguage' => 'ru',
            'fileMap' => [
                'app' => 'app.php',
                'app/error' => 'error.php',
            ],
        ],
    ],
],
```