oid_object_info(): drop type_name strbuf

oid_object_info(): drop type_name strbuf

We provide a mechanism for callers to get the object type as a raw
string, rather than an object_type enum. This was in theory useful for
returning types that are not representable in the enum, but we consider
any such type to be an error, and there are no callers that use the
strbuf anymore.

Let's drop support to simplify the code a bit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>