[feedback-report] remove unused feedback types

[J.D. Falk]

It's a pretty safe bet that Return Path is among the top three generators of 
ARF messages (counting by message), and we only use the 'abuse' type -- yet 
I still see confusion about the others.  This leads to interoperability 
issues, such as when a feedback consumer assumes that they'll only have to 
care about "opt-out" while pretty much all of the feedback providers in the 
world only provide "abuse".

So, to kick off the first argument as we move into the possible creation of 
a formal IETF WG, I propose that we:

Keep: abuse, dkim, not-spam, other
Drop: fraud, miscategorized, opt-out, virus

And keep the registry, so that new types may be added in the future if a 
demonstrable need arises.

What say you?

-- 
J.D. Falk
Return Path Inc
http://www.returnpath.net/