repo: Further optimize `ostree_repo_list_objects_set()`

repo: Further optimize `ostree_repo_list_objects_set()`

In a prior change we discovered that for bad historical reasons
libostree was returning a mapping "object type+checksum" => "metadata"
but the "metadata" was redundant and pointless.

Optimize the prune API to use a (currently internal) object listing
API which returns a set, not a map.  This allows `GHashTable` to
avoid allocating a separate array for the values, neatly cutting
memory usage in half (from ~13MB to ~6MB) on my test case of a
dry-run prune of a FCOS build.