hash: require hash algorithm in `empty_tree_oid_hex()`

hash: require hash algorithm in `empty_tree_oid_hex()`

The `empty_tree_oid_hex()` function use `the_repository` to derive the
hash function that shall be used. Require callers to pass in the hash
algorithm to get rid of this implicit dependency.

While at it, remove the unused `empty_blob_oid_hex()` function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>