tree-wide: Use helpers for unlinkat()

We have `ot_ensure_unlinked_at()` for the "ignore ENOENT" case, and
`glnx_unlinkat()` otherwise. Port all in-tree callers to one or the other as
appropriate.

Just noticed an unprefixed error in the refs case and decided to do a tree-wide
check.

Closes: #1142
Approved by: jlebon

src/libostree/ostree-repo-checkout.c

@@ -1121,11 +1121,8 @@ ostree_repo_checkout_gc (OstreeRepo        *self,
 
           if (stbuf.st_nlink == 1)
             {
-              if (unlinkat (dfd_iter.fd, dent->d_name, 0) != 0)
-                {
-                  glnx_set_error_from_errno (error);
-                  return FALSE;
-                }
+              if (!glnx_unlinkat (dfd_iter.fd, dent->d_name, 0, error))
+                return FALSE;
             }
         }
     }