Function delete_files 

pub async fn delete_files(
    region_id: RegionId,
    file_ids: &[(FileId, u64)],
    delete_index: bool,
    access_layer: &AccessLayerRef,
    cache_manager: &Option<CacheManagerRef>,
) -> Result<()>

Delete files for a region.

• region_id: Region id.
• file_ids: List of (file id, index version) tuples to delete.
• delete_index: Whether to delete the index file from the cache.
• access_layer: Access layer to delete files.
• cache_manager: Cache manager to remove files from cache.