remove heap implementation in dampening.c

The current dampening patches uses two different implementations at the
same time, quote from
https://lists.dns-oarc.net/pipermail/dns-operations/2012-October/009202.html:

"There are two implemtations activated in the patch: The (old) heap and
a
(new) hash/queue. I do not know which of them performs better so I run
both
in parallel to gather information. If you do not like such a play,
please
remove the approbiate init function from "implementations" in
dampening.c."

Quote from
http://lutz.donnerhacke.de/eng/Blog/DNS-Dampening-under-the-microscope:

"Conclusion: The heap is more efficient, but the queue performs better.
On FreeBSD the heap implementation causes segfaults for still unknown
reasons. The queue is working without any problems."

Hence I have disabled the heap impmentation in this patchset as it is
for FreeBSD :)