Do not delete items on read-only index

Issue

On `src/Plugin/search_api/backend/SearchApiSolrBackend.php` function `deleteAllIndexItems` we are not checking that the index is read-only.

In this file from `search_api` module, `search_api/src/Entity/Server.php`, we check the read-only property. See `deleteAllIndexItems`.

Solution

Add code to check the read-only property in `Drupal\search_api_solr\Plugin\search_api\backend::deleteAllIndexItems`, as in `Drupal\search_api\Entity\Server::deleteAllIndexItems`.
```
if ($index->isReadOnly()) {
$vars = [
'%index' => $index->label(),
];
$this->getLogger()->warning('Trying to delete items from index %index which is marked as read-only.', $vars);
return;
}
```