repo: Drop the branch parameter from ostree_repo_commit

It's unused. Make users explicitly write a ref if they want this;
high-level convenience API will be introduced later.

https://bugzilla.gnome.org/show_bug.cgi?id=707727

src/libostree/ostree-repo.h

@@ -315,7 +315,6 @@ gboolean      ostree_repo_write_mtree (OstreeRepo         *self,
                                        GError            **error);
 
 gboolean      ostree_repo_write_commit (OstreeRepo   *self,
-                                        const char   *branch,
                                         const char   *parent,
                                         const char   *subject,
                                         const char   *body,