odb: adjust naming to free object sources

odb: adjust naming to free object sources

The functions `free_object_directory()` and `free_object_directories()`
are responsible for freeing a single object source or all object sources
connected to an object database, respectively. The associated structure
has been renamed from `struct object_directory` to `struct odb_source`
in a1e2581a1e (object-store: rename `object_directory` to `odb_source`,
2025-07-01) though, so the names are somewhat stale nowadays.

Rename them to mention the new struct name instead. Furthermore, while
at it, adapt them to our modern naming schema where we first have the
subject followed by a verb.

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