hs: Forgotten log_warn for prop224 intro point

Turns out we forgot to remove that log statement from hs_intropoint.c:

  log_warn(LD_GENERAL, "Established prop224 intro point on circuit %" PRIu32,
           circ->p_circ_id);

It has been reported here: https://lists.torproject.org/pipermail/tor-relays/2017-August/012689.html

As far as I can tell, it seems the only warning that is misplaced and that one is unneeded.

However, I might want to take the opportunity to change 3 log_warn() and put them to protocol warning level instead.

    log_warn(LD_REND, "Unable to send INTRODUCE2 cell to the service.");
...
    log_warn(LD_REND, "Unable to send an INTRODUCE ACK status %d to client.",
             status);
...
    log_warn(LD_BUG, "Couldn't send INTRO_ESTABLISHED cell.");

They've been introduced in tor-0.3.0.1-alpha and tor-0.3.0.2-alpha.