libglnx porting: Use glnx_shutil_rm_rf_at()

In some cases (such as `ostree-sysroot-cleanup.c`), the surrounding
code would be substantially cleaner if it was also ported to
fd-relative, but I'm going to do that in a separate patch.

That way these patches are easier to review for mechanical
correctness.  I used an Emacs keyboard macro as the poor man's
[Coccinelle](http://coccinelle.lip6.fr/).

src/libostree/ostree-repo-prune.c

@@ -236,7 +236,7 @@ ostree_repo_prune_static_deltas (OstreeRepo *self, const char *commit,
 
       deltadir = _ostree_get_relative_static_delta_path (from, to, NULL);
 
-      if (!gs_shutil_rm_rf_at (self->repo_dir_fd, deltadir,
+      if (!glnx_shutil_rm_rf_at (self->repo_dir_fd, deltadir,
                                cancellable, error))
         goto out;
     }