Rename ADDON_ICON_SIZES and only generate the ones we need (bug 885329)

https://bugzilla.mozilla.org/show_bug.cgi?id=885329

We should probably deprecate 32x32 and 48x48 as well, but that's for another bug.
I didn't know/expect `webpay_jwt.py` to expose addon icons sizes like this. Since I'm removing 2 sizes we never exposed in the regular API, will that be a problem ?