repo: Delete the last use of GFile tmp_dir

The keyring isn't large, so let's just fall back to copying it
rather than requiring `renameat()`.

Prep for `ostree_repo_open_at()`.

Closes: #821
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -78,7 +78,6 @@ struct OstreeRepo {
 
   GFile *repodir;
   int    repo_dir_fd;
-  GFile *tmp_dir;
   int    tmp_dir_fd;
   int    cache_dir_fd;
   char  *cache_dir;