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.c

@@ -1401,8 +1401,6 @@ ostree_repo_stage_commit (OstreeRepo *self,
                           const char   *parent,
                           const char   *subject,
                           const char   *body,
-                          GVariant     *metadata,
-                          GVariant     *related_objects,
                           const char   *root_contents_checksum,
                           const char   *root_metadata_checksum,
                           char        **out_commit,
@@ -1422,9 +1420,9 @@ ostree_repo_stage_commit (OstreeRepo *self,
 
   now = g_date_time_new_now_utc ();
   commit = g_variant_new ("(@a{sv}@ay@a(say)sst@ay@ay)",
-                          metadata ? metadata : create_empty_gvariant_dict (),
+                          create_empty_gvariant_dict (),
                           parent ? ostree_checksum_to_bytes_v (parent) : ot_gvariant_new_bytearray (NULL, 0),
-                          related_objects ? related_objects : g_variant_new_array (G_VARIANT_TYPE ("(say)"), NULL, 0),
+                          g_variant_new_array (G_VARIANT_TYPE ("(say)"), NULL, 0),
                           subject, body ? body : "",
                           GUINT64_TO_BE (g_date_time_to_unix (now)),
                           ostree_checksum_to_bytes_v (root_contents_checksum),

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,