tree-wide: Remove unused variables detected by CLang

CLang finds these, whereas GCC treats having
`__attribute__((cleanup))` as a use.

This obsoletes https://github.com/ostreedev/ostree/pull/411

Closes: #548
Approved by: jlebon

src/libostree/ostree-fetcher.c

@@ -1282,7 +1282,6 @@ _ostree_fetcher_mirrored_request_to_membuf (OstreeFetcher  *fetcher,
 {
   gboolean ret = FALSE;
   const guint8 nulchar = 0;
-  g_autofree char *ret_contents = NULL;
   g_autoptr(GMemoryOutputStream) buf = NULL;
   g_autoptr(GMainContext) mainctx = NULL;
   FetchUriSyncData data;