call self::getNumericStorageId() to get the numeric ID

lib/private/files/cache/storage.php

@@ -101,7 +101,7 @@ class Storage {
 		$sql = 'DELETE FROM `*PREFIX*storages` WHERE `id` = ?';
 		\OC_DB::executeAudited($sql, array($storageId));
 
-		$numericId = self::exists($storageId);
+		$numericId = self::getNumericStorageId($storageId);
 		if (!is_null($numericId)) {
 			$sql = 'DELETE FROM `*PREFIX*filecache` WHERE `storage` = ?';
 			\OC_DB::executeAudited($sql, array($numericId));