from_disk: Update loader classes implementation
GitLoaderFromDisk
and GitLoaderFromArchive
implementation can now
be simplified by inheriting from GitLoader
class and by overriding
the fetch_pack_from_origin
method to fetch git objects from disk
instead of fetching them from a remote URL.
Also remove the deprecation notice for GitLoaderFromDisk
as it is
likely there will be use cases in a near future for that git loader.
Related to !190 (comment 183093)