remove a DPRINTF() from iwm(4) which no longer makes any sense

remove a DPRINTF() from iwm(4) which no longer makes any sense

The conditions under which this debug message should trigger were
changed in r1.330 (cvs commit RcatFKLXoHUopLvW), which removed a
txd_done() call located before txq_advance(). The mbuf for the
most recently completed frame will no longer be NULL on entry.

Prompted by a question from waddlesplash at haiku-os.