Drop support for related objects and metadata in commit objects

While the actual commit object format is presently the same, for a
number of reasons we'd like to change it fairly radically.  Among
other things, we need to drop our a{sv} types in objects, to protect
against GVariant changing format.

Since now gnome-ostree now longer uses related objects, and nothing
ever used metadata, just drop them both.

src/libostree/ostree-repo.h

@@ -253,8 +253,6 @@ gboolean      ostree_repo_stage_commit (OstreeRepo   *self,
                                         const char   *parent,
                                         const char   *subject,
                                         const char   *body,
-                                        GVariant     *metadata,
-                                        GVariant     *related_objects,
                                         const char   *content_checksum,
                                         const char   *metadata_checksum,
                                         char        **out_commit,