Revert if_oqdrops accounting changes done in kernel, per request from mpi@.

Revert if_oqdrops accounting changes done in kernel, per request from mpi@.

(Especially adding IF_DROP() after IFQ_ENQUEUE() was completely wrong because
IFQ_ENQUEUE() already does it.  Oops.)

After this revert, the situation becomes:

- if_snd.ifq_drops is incremented in either IFQ_ENQUEUE() or IF_DROP(), but
  it is not shown to userland, and

- if_data.ifi_oqdrops is shown to userland, but it is not incremented by
  anyone.