remove default verb

[willnorris]

Having a default verb made sense for the Atom serialization because then we were able to say "all valid Atom feeds are also Activity Streams". All activities have a verb, it's just that it is sometimes implied. I'm not sure that there is any value in having a default verb for the JSON serialization, since we're not trying to support an existing format like Atom.

[willnorris]

looks like I misread the "implied post activity" section. It's not defining an implied verb, it's defining an implied activity, who's verb is "post".