remove add-on-specific fields from `Contribution` (bug 1026281)

https://bugzilla.mozilla.org/show_bug.cgi?id=1026281

r? @andym @diox @robhudson
I've been sticking bigger migrations like these in https://etherpad.mozilla.org/bug-999119 and committing them the week after they're pushed to prod to avoid any errors during push.