fix postponed sorting assertion failure

The Postponed Dialog doesn't support sorting.

We use a MailboxView to hold the Mailbox and number the Emails.
The MailboxView tries to sort the Emails and fails.
The assertion check isn't necessary.

---

Steps to reproduce:
    Create a mailbox 'p.mbox' with some emails in it.

test.rc
    set postponed   = p.mbox
    set sort        = threads
    set use_threads = threads

Run:
    ./neomutt -n -F test.rc -p

mutt_thread.c

@@ -796,7 +796,6 @@ static void mutt_sort_subthreads(struct ThreadsContext *tctx, bool init)
   if ((c_sort & SORT_MASK) == EMAIL_SORT_THREADS)
   {
     ASSERT(!(c_sort & SORT_REVERSE) != reverse);
-    ASSERT(cs_subset_enum(NeoMutt->sub, "use_threads") == UT_UNSET);
     c_sort = c_sort_aux;
   }
   c_sort ^= SORT_REVERSE;